aurance
Explorer
Frankly, just give out +1 every 3 levels and be done with it.
You could do that, and generally it'll work out fine, although then you'll have some levels where PCS increase hit by +2 at once, due to ability score increases. I've moved the numbers around a bit so this doesn't happen, and also included a progression for heavy armor that obviates the need for the clunky masterwork mechanic.