It depends on whether you are going for "realism" or "simplicity" Option one goes for the former, option two goes for the latter.
Option 1: Piggy-backing on what you already have in other lists. If it duplicates an Evoke spell, then use the Evoke damage cap. (Whether you would have the same damage cap., or an expanded damage cap. to reflect that only 20-60% of the damage gets through, is up to you -- also, would the lesser damage getting through allow a save vs. illusion, or would the person just think she was lucky?). This solves the "mass vs. individual" problem, I think.
And how would this be roleplayed? Would you have the victim believe that they took 100% of the damage until they make a save or the fight is over? If they would have gone unconscious/died if the damage were actually 100%, what happens? Or do they realize that they are only taking the damage they are actually taking (the 20%-60%), even if they fail their save/not get to roll a save?
If it duplicates a Summon spell, use that list you have under Summon to cap the CR, and hopefully the corresponding damage (this is if it is partially real. If it is non-real, then there are no limits, to quote the d02 system designer).
Again, you would have to decide whether the 20%-60% real cap means that you could "summon" creatures of higher CR (as a poster put above). On the one hand it would be nice - but you don't want to make illusion lists with reality either more or less desireable than the relevant Evoke/Create/Summon spells.
The way I would see it, the illusionist gains versitility, but on the other hand a good evoker/summoner/creator will already have that versility if they have enough Evoke/Summon/Create spells. Mind you the illusionist has versitility between whether the illusion will be "evoke", "summon" or "create". The illusionist gains "instant friendliness" over the summoner, and could also "summon" someone from the plane she is on, which the summoner can't do.
If it duplicates a Create spell, it can't cause damage directly (but could indirectly). Here the 20%-60% of what can be Created might fall down a bit, since the Create list cap is related to gp cost, and burning lava is relatively cheap.
(Actually, how do you cost out burning lava? or a pit (a la illusion duplicating create void? or illusion space/illusion force? or how do you make a fake pit anyhow?)?) Maybe use the Evoke cap for damage again, if the object should indirectly cause damage. Or maybe this would be the time to use the simpler "per round" rule below:
Option 2: Say that the particular spell/monster evoked is irrelevant -- a victim takes 20%-60% of X damage per round (force damage?), where X is a ceiling controlled by the MP of the spell. If there is more than one victim, then the caster can spread out this damage (1d6 for victim A, 2d6 for victim B, or else roll up the total damage and allocate the hp of damage as one sees fit, but that might give too much control to the mage -- or be nicer to the victims and say the person with the lowest will save/wis/int, in that order, takes all the damage coming to the group, but then the others in the group would know it wasn't real since they should have taken damage. hmmm....). Or you can be nice to the caster by having each victim in a "mass" attack take the percentage of X damage. The advantage of this simpler method is that the caster would not need stats for the "Creatures" it "summons" or "evoke lists" it "uses" or even indirectly the "create" spells it "uses". The relevant creature/evoke/created indirect damage effect just "hits" and does that partial damage. And then the cap and the % loss, could be justified by being "auto damage" each round (assuming you have illusion Life in the mix to let the illusion keep acting on its own) or could be "one round only" or "spread out over rounds" or what have you.
Or else, looking at your thoughts, maybe have a "Damage pool" of xd6 (=MP?) that the spell could cause, and this can be spread out over many rounds or done "all at once", regardless of the illusion? Maybe the simple/standard/complex could be used (instead of as a %) to determine how many people, max., could take this damage. So a simple spell with a cap of 5d6 does either 5d6 to one person and nothing else ever, or 2d6 to one person one round, 3d6 to a second person a second round, and nothing else ever. The same spell, if standard, can to 5d6 to 3 people, and nothing else ever, or be more spread out, but could not do more than 5d6 to one person in one round. The same spell, if complex, can do 5d6 to 5 people, using the same idea. And the 1-3-5 thing is just my first thought -- you might want to adjust it. The 5d6 cap was similarly pulled out of the air for the sake of the example. The nice thing here is that the damage does not change, whether it is a "Fireball", or "demon" or "burning lava". Note: if you want someone to take damage, you have to "justify" it by having your illusion "hit" that person or otherwise seem to affect that person. But if someone "should" have been damaged but is allocated no damage (or perhaps "should" have been completely immune (but I am immune to fireballs!), and yet take damage!), they can roll a will save, as per your rules on p. 65. But you can tailor this, as an illusionist (if you want person A to take the damage instead of person B, have the demon "miss" person B, or the fireball only "wash over" person A, or the lava, er, flow around person B briefly, for whatever reason (a sudden air bubble in the lava or something).
Note on the simpler methods, the damage is likely to be automatic, so that would be an argument for keeping it low. If you allow "Saves" for half or no damage vs. this damage, or "defence" or "dodge" rolls or whatever, then the damage cap should be correspondingly higher. How low or high I leave up to you.
Or, instead of create force as a prereq., just have evoke force as a prereq, and have the damage entirely separate from the illusion, but seeming to come from the illusion. I guess it depends on what you want the 20th level illusion caster to be able to do, compared to the 20th level evoke caster and the 20th level summon caster and the 20th level create caster.
Also, is the damage of a particular type, like Force (or is it whatever you are duplicating -- fire, acid, etc.?) or is it untyped? This might be relevant for special resistances, and maybe that 1.5x damage on some types.
While we are at it, there is a cut-and-paste typo for the top of the 2nd column on page 21. I assume it is under the art?
And to clarify, if one has both spell list familiarity (Evoke Death) and spell list familiarity (Hex Life) and casts the Evoke Death 7/Hex Life 10/Gen 4 spell, it would be 2 MP cheaper (one for each spell list familiarity involved in the spell)? This is the feat on p. 16).
And back to illusions, under Resisting Illusions on p. 65, you say "Additionally, if a creature is "hit" by an illusion and should take damage, it automatically makes a will save to disbelieve, unless you included an Illusion Force component to your spell. If it fails, it may still suspect an illusion, or it might think the illusion is incorporeal or somehow else unable to affect him". I would suggest, for clarity, that you replace the words "makes a will save" with the words "rolls a will save", since I assume by the following sentence that you don't intend creatures "hit" by a non-force illusion to automatically succeed at a will save.
Anyhow, best of luck. And I do like your work. I wouldn't be so picky if it wasn't worth bothering over.
