I suspect you're ignoring the clamp on the BaseToHit + ToHitBuffs - ToHitDebuffs - (Defense - DefenseDebuffs) bit.
However, due to feeling like suck, beinf depressed over various things, and not feeling like working it out, unless you give a worked example, I don't think I can know for sure.
This might help http://wiki.cohtitan.com/wiki/Attack_Mechanics though it is not the best explanation of the mechanism I've ever seen.
Disclaimer: The above may be humerous, or at least may be an attempt at humour. Try reading it that way.
Posts are OOC unless noted to be IC, or in an IC thread.
I think you forgot to calculate the Defense DeBuff the higher level mobs have.
- @DSorrow - alts on Union and Freedom mostly -
Currently playing as Castigation on Freedom
My Katana/Inv Guide
Anyone who doesn't take truth seriously in small matters cannot be trusted in large ones either. -Einstein
Thanks for the replies 
I don't think the clamp comes into these examples because they're always in the 5-95% range.
DSorrow, did you mean defense debuff resistance?
I wondered about that but the examples I ran through didn't give consistent values for the resistance - perhaps I miscalculated though. It would explain why the debuff affected +3s less than +2s.
a couple of worked examples:
1. Melt Armor - defense debuff enhanced to 38.36%
Decoy Phantasm against a +3 with no defense
NetToHit = (1) * (1 + 0) * [ 0.48 + 0 - 0 - (0 - 0) ]
NetToHit = 0.48
which matches what I observed
the same with Melt Armor in effect
NetToHit = (1) * (1 + 0) * [ 0.48 + 0 - 0 - (0 - 0.3836) ]
NetToHit = 0.8636
but I observed 0.7293
2. Slash - defense debuff 7.5%
Strike against a +3 with no defense
NetToHit = (1) * (1 + 0.577) * [ 0.48 + 0 - 0 - (0 - 0) ]
NetToHit = 0.75696
which matches what I observed
the same with Slash in effect
NetToHit = (1) * (1 + 0.577) * [ 0.48 + 0 - 0 - (0 - 0.075) ]
NetToHit = 0.875235
but I observed 0.8338
When you say +2 and +3, this is just level, and just minions is it?
Disclaimer: The above may be humerous, or at least may be an attempt at humour. Try reading it that way.
Posts are OOC unless noted to be IC, or in an IC thread.
Yes, +2 and +3 to my (or pet's) level. I don't believe rank affects a player's chance to hit them.
It seems the info I was missing was:
+0 "75% chance to hit and your powers are 100% effective"
+1 "65% chance to hit and your powers are 90% effective "
+2 "56% chance to hit and your powers are 80% effective "
+3 "48% chance to hit and your powers are 65% effective "
It all adds up now
When a defense debuff is involved the Last Hit Chance values I've seen don't match what I calculate - can anyone explain how it works please?
I noted the Last Hit Chance values for Decoy Phantasm, before & after Melt Armor affected the target.
One benefit of using Decoy Phantasm for this is it's not affected by accuracy enhancements so has the base 56% chance to hit +2s & 48% chance to hit +3s.
Melt Armor is enhanced to a 38.36% defense debuff (from 20%).
From Arcana's Guide to Defense:
NetToHit = (InherentAttackAccuracy) * (1 + AccuracyEnhancement) * [ BaseToHit + ToHitBuffs - ToHitDebuffs - (Defense - DefenseDebuffs) ]
I expected 38.36 to basically add to BaseToHit but the results (against a variety of targets) were always:
against +2s
base to-hit 56%
to-hit after MA 86.68%
so MA meant +30.68%
but expected +38.36%
against +3s
base to-hit 48%
to-hit after MA 72.93%
so MA meant +24.93%
but expected +38.36%
Any ideas how this works?