I'm not yet to the point where I may advocate how to attribute XP to players according to encounter difficulty. I leave this for the DMG as I will probably use the milestone system for myself.
One word about the last suggestion by [MENTION=6688277]Sadras[/MENTION]. I'm quite puzzled by the thresholds in the Basic rules to define "easy", "moderate" and "difficult" with respect to "deadly". Levels 1, 2 and 4 apply one formula (25%, 50% and 75%), Level 3 another (18.7%, 37.5%, 56.3%), and all other levels go for (22%,44%,66%). I've no clue where it is coming from, and without any further definition of what "medium" means, I sticked to the (25%,50%,75%) scheme.
The (40%,60%,80%) for PEL I came with in the OP is just the translation of these (25%,50%,75%) for XP. So if you go for XP, you have to use the XP percentages.
I am not sure about whether PEL works well with mismatched groups of monsters (high CR plus low CR) because I have not run much in the way of those mixed monster groups. Although I will be running one next week.
As a suggestion to [MENTION=6780929]Gobelure[/MENTION], is there a way to get the results in the normal XP budget numbers? The thing that really shines in this method is the ramping up of monster difficulty. Since the party is only supposed to get the base (not the multiplied) xp of the monsters could you apply your multiplier to the XP required by the party and the XP given by the monsters (i.e. pull the PEL numbers out)?
Thanks very much for all your suggestions and discussion, it made me improve my own understanding. Of course, the mismatched CR test is the one to be done in order to playtest PEL vs. XP.
You mentioned that my system sets usually an easier difficulty for encounter than the official one. I'm looking out where it is coming from and will come with some answers to that.
Now, you asked for a formula to revert back to encounter XP: this is the reverse process that led from XP to PEL.
XP = (PEL^1.5) * 50
In practice,
1) take the total PEL
2) in the Monster table, look for the CR equivalent of this PEL
3) in the DMG table, get the XP for a monster of that CR, doing any necessary interpolation.
Or use the following table: once you get your total PEL, you can have to the right column the corresponding enounter XP. I only listed those corresponding to an equivalent monster CR, so you still have to guess numbers which fall between 2 rows.
[table="width: 300, class: grid, align: center"]
[tr]
[td]
Equivalent Monster CR[/td]
[td]
Total PEL[/td]
[td]
Encounter XP[/td]
[/tr]
[tr]
[td]0[/td]
[td]1/3[/td]
[td]10[/td]
[/tr]
[tr]
[td]1/8[/td]
[td]2/3[/td]
[td]25[/td]
[/tr]
[tr]
[td]1/4[/td]
[td]1[/td]
[td]50[/td]
[/tr]
[tr]
[td]1/2[/td]
[td]1.5[/td]
[td]100[/td]
[/tr]
[tr]
[td]1[/td]
[td]2.5[/td]
[td]200[/td]
[/tr]
[tr]
[td]2[/td]
[td]4[/td]
[td]450[/td]
[/tr]
[tr]
[td]3[/td]
[td]6[/td]
[td]700[/td]
[/tr]
[tr]
[td]4[/td]
[td]8[/td]
[td]1,100[/td]
[/tr]
[tr]
[td]5[/td]
[td]11[/td]
[td]1,800[/td]
[/tr]
[tr]
[td]6[/td]
[td]13[/td]
[td]2,300[/td]
[/tr]
[tr]
[td]7[/td]
[td]15[/td]
[td]2,900[/td]
[/tr]
[tr]
[td]8[/td]
[td]18[/td]
[td]3,900[/td]
[/tr]
[tr]
[td]9[/td]
[td]21[/td]
[td]5,000[/td]
[/tr]
[tr]
[td]10[/td]
[td]24[/td]
[td]5,900[/td]
[/tr]
[tr]
[td]11[/td]
[td]28[/td]
[td]7,200[/td]
[/tr]
[tr]
[td]12[/td]
[td]32[/td]
[td]8,400[/td]
[/tr]
[tr]
[td]13[/td]
[td]36[/td]
[td]10,000[/td]
[/tr]
[tr]
[td]14[/td]
[td]40[/td]
[td]11,500[/td]
[/tr]
[tr]
[td]15[/td]
[td]44[/td]
[td]13,000[/td]
[/tr]
[tr]
[td]16[/td]
[td]48[/td]
[td]15,000[/td]
[/tr]
[tr]
[td]17[/td]
[td]52[/td]
[td]18,000[/td]
[/tr]
[tr]
[td]18[/td]
[td]56[/td]
[td]20,000[/td]
[/tr]
[tr]
[td]19[/td]
[td]60[/td]
[td]22,000[/td]
[/tr]
[tr]
[td]20[/td]
[td]64[/td]
[td]25,000[/td]
[/tr]
[tr]
[td]21[/td]
[td]76[/td]
[td]33,000[/td]
[/tr]
[tr]
[td]22[/td]
[td]88[/td]
[td]41,000[/td]
[/tr]
[tr]
[td]23[/td]
[td]104[/td]
[td]50,000[/td]
[/tr]
[tr]
[td]24[/td]
[td]120[/td]
[td]62,000[/td]
[/tr]
[tr]
[td]25[/td]
[td]136[/td]
[td]75,000[/td]
[/tr]
[tr]
[td]26[/td]
[td]152[/td]
[td]90,000[/td]
[/tr]
[tr]
[td]27[/td]
[td]168[/td]
[td]105,000[/td]
[/tr]
[tr]
[td]28[/td]
[td]184[/td]
[td]120,000[/td]
[/tr]
[tr]
[td]29[/td]
[td]200[/td]
[td]135,000[/td]
[/tr]
[tr]
[td]30[/td]
[td]216[/td]
[td]155,000[/td]
[/tr]
[/table]