Just so I'm clear on what you are saying - Is your premise that a monster should do about 1/2 PC hps in damage over 3 rounds (with adjustments for elites, solos and minions?
Well, 2 surges worth of damage before considering what healing or mitigation (shield, temp, etc) abilities the party has. At a certain point, you need a certain level of threat or there's just no reason to have the fight. As it is, if you had a party of 4 face a group of 4 monsters that triggered LOW, you would not be surprised if a party might not have to expend any healing triggers whatsoever to win the combat (no chance of anyone dropping), that it's possible that no one would even get bloodied depending on how the damage was spread or mitigated (battlerager, bard, or barbarian temp hp).
Edit - With basic stabs that is...
Well, using every single ability they had that seemed to affect damage. I probably should give a stronger weighting to dazing someone, in terms of improving the damage output of other monsters, but that's so very fuzzy.
It is built on a lot of assumptions - for example, if you have 5 normal monsters and kill 1 each round through focus fire, then they might get off 5 attacks the first round, 4 the second, 3, 2, 1 for a total of 15 attacks... which averages out to 3. Of course, some critters are just going to get priority attacking over others (why, hello, goblin hexer) and others are just going to cheat that amount of time (why, hello, wraith). But that's okay, cause you need to account for what happens if something is not focus fired. Some fights the creatures use their daze/immobilize/stun abilities over their damage ones, but that can wash out between lost player damage and lost monster damage frankly.
Once I've got this output spreadsheet going reasonably I intend to make an input one to measure creatures that are excessively easy or hard to kill. So, wraiths that have in theory (and more actuality than I care to think about) infinite hp can go on there. Some creatures who have defenses way too low will show up too. I actually figured at some point in this I'd adjust the factor for things like artillery and lurkers that are a bit more fragile, giving them more leeway to do higher damage (and more stringent on low damage) as a result.
I wanted to do this mostly so I could have somewhere to sanity check my own stuff so I could tool around more comfortably with the monster builder, and because I'm thinking of redoing the monster manual (or possibly just its most broken parts) for fun.