Other:
Messed around with the Marine Damage Resistance formula a lot and determined that it works fine as is. Variants tried involved different Damage Multiplier, removing half the armor from the starting damage and end damage, and so on. If anyone wishes to mathhammer it themselves and try to find a better formula it currently works as follows:
-Damage Multiplier: 0.7 - ((Marine Experience * Marine Experience) /40000), minimum of 0.25 Displayed damage resistance is ((1 - Damage Multiplier) * 100)%
-Damage Result: (Damage * Damage Multiplier) - Marine Armor
Just to help brainstorm how combat could work (I really have no idea how it does work right now): It's always been a mainstay of Warhammer combat mechanics that armor is relative - the harder you're hit, the more damage your armor soaks. This means the above arithmetic betrays that fundamental idea; one immediate contradiction is that good enough armor theoretically reduces incoming damage to 0 (which warhammer armor can't do).
The extent to which you want to model warhammer combat is probably iffy, because the tabletop includes some really bizarre rules interactions like cover making you more damage resistant in a way ballistic skill can't fix, even though allegedly most cover just makes you harder to hit.
Here's some base math for simulating Warhammer combat, fixing as little wackiness as possible, and skipping the step for rolling to hit:
marine hit points = wounds on profile * 46656//This is 6**6, enough to cope with rolling to wound, save, and FNP, with or without rerolls on any of them.
Basic combat (no rerolling, marine is T4, W1, Sv 3, FNP -):
svt = max(min(strength-toughness+3,5),min(strength-toughness+4,1),0)//Extra min is because you can still wound on 6+ against s+3.
if(armor<penetration){realarmor=armor-1}else{realarmor=6}//2..7 scale, where 7 is armor or AP -
svta = realarmor*svt//Odds of overcoming toughness times overcoming armor
svta *= 6**4 //missing pieces: rerolling failed wounds, failed saves, and both base and rerolled FNP; this result is how much damage the marine takes from the hit.
It should be obvious how to work in instant kill at this point, but many special rules cause all sorts of checks in the above logic, so I left them out. All this does is apply expected probability of wounding as static values, so you always deal the same damage rather than rolling.
The above rules are kind of awful, as I mentioned (for example, I left it out, but you'd apply the better of armor and cover, rather than having them stack and apply to different steps in the attack process, like we expect in every other game, Dark Heresy included). If you're your own Chapter Master, you're also going to want to be able to do things like give your Scouts Storm Shields, at which point the power armor becomes a waste of time under the tabletop rules. Skipping the intermediary logic, I suggest this for a shooting attack:
hit points = wounds*(6**
//Now including to-hit, due to incorporating a marine being harder to hit
hitpiece = 6*max(ballistic-cover,1)//It's always possible to hit, but ballistic skill and cover duke it out in terms of real odds; very high ballistic skill overflows into excess damage.
//Note on Cover here: count up from 0, so 0 is cover - and 5 is cover 2+.
//Extra 6 is because we're not doing rerolls to hit.
woundpiece = 6*hitpiece*max(strength-toughness+3,min(strength-toughness+4,1),0)//Very high strength now elegantly overflows into damage, precluding any need for a manual rule about instant killing when s=2t.
//Extra 6 is because we're not doing rerolls to wound.
armorpiece = 6*woundpiece*(6-max(armor-penetration,0))//armor now degrades with penetration, like strength does with toughness and bs now does with cover.
//Note on Armor and Penetration: count up from 0, so 0 is - and 5 is 2+ armor or AP 2.
//Extra 6 is because we're not doing rerolls on armor save.
And so on. I would suggest invulnerability as an additional save after armor (you'll need to explode out the hit point total again, if you want to be able to cope with reroll mechanics on it), with Feel No Pain forcing rerolls on successful wounds rather than the current rule of it being the additional save.