Perfect...I didn't realize there were different base armors aside from the +4 and +6 specializations in PHB. Odd considering I own Adventurer's Vault 1&2. Thanks for the clarification!
On the basis of your breakdown of the manual calculation, it appears you're using inherent bonuses to defences, in which case you're not using magical armour - only non-magical chainmail (which cannot be masterwork). Now I'm confused as to why "ah you forgot the masterwork armour bonus" is the answer when you indicated you weren't using magical armour...
Cheers, Al'Kelhar