Level + 3 sounds nice... I was looking for a math thread with MM3 + Essentials monsters as a base but can only find earlier stuff. Anybody point me to anything else?
Level + 3 (to hit) pretty much covers:
1/2 level mod (+15 at level 30)
+1 every 1/2 tier via enchantment (+6 at 30)
+1 to attack ability (assumed) at 8, 14, 21 and 28 (+4 at 30)
+1 per tier via expertise (+3 at 30)
That's +28 difference between 30 and 1, so it's pretty much works out to scaling by level if you eliminate those other sources of attack modifiers. There are other boosts (epic destinies that give an extra +2 to stats, paragon paths that give boost to attacks, etc).
Now, for damage, you have other factors. You are similarly starting with the 4 at level 1 (Level + 1) but you don't get the 1/2 level mod. So if you bake in just the enchantments and ability boosts, you'd get +10 over the course of 30 levels. So, basically 4 + 1/3 for damage. This still leaves stuff like weapon/implement focus and other damage boosting feats/items open. It does, however, require figuring out stuff like Twin Strike that does [W] with no ability mod, since the enchantment bonus is baked into the ability mod damage. [Ditto stuff that does ability mod damage but doesn't bake in enchantment bonuses like Furious Smash]. There would still be other options to boost damage, like focus, twf, etc. If you bake in the weapon focus (and nothing else) you can get it up to closer to 4 +1/2 level which is a bit cleaner than 1/3.
For Defenses:
With AC, you'd leave the shields alone ... they do a fixed bonus that doesn't scale up, so it's simple to add on top of other modifiers.
Heavy armor:
You start with 16-18 depending on the armor. You add, over the course of 30 levels:
+15 via 1/2 level mod
+6 via enchantment
+1 via armor spec at 11
+6 via masterwork qualities [assuming you take the one that boosts AC only]
For +28 over 30 levels. So a 15+1 per level in chain, 16+1 per level in scale, 17+1 per level in plate is approximately right, with shields (but not shield spec) allowed, and other class features, etc
For light armor, you would probably go with 10, 12 or 13 + abilty mod, so assuming the base +4 assumption we had been working on, means 14, 16 or 17 at level 1.
+15 via 1/2 level mod
+6 via enchantment
+1 armor spec at 11 (true, it only really counts for hide)
+4 via ability boosts (assuming you improve it at every opportunity)
+2 via masterwork (assuming you take the one that boosts AC only)
That's +28 again, so 13 + level in cloth, 15+ level in leather and 16 + level in hide are all options. This does mean that leather and chain are equivalent, which does hurt chain as an option by removing the benefits it has over leather but leaving it's penalties to skill and speed.
For NADs:
You get, over the course of the levels:
+15 via half level
+3 via improved defenses
+4 (to two of them) via ability score increase
+1, maybe +2 if it started with an odd number (to one) via automatic ability score increase
+6 via enchantment
So, the best case is +28, similar to the other ones.
For the bad defense, it gets effectively +25, which can be made up via one of the Epic Defense feats which give a +4 untyped bonus to a defense.
In that case, it can be a 13 + level to two defenses and 11 + level to a third (assuming a 18/18/12/10/10/10 ability spread) with bonuses via class, but stuff like the Belt of Vim (+1 to a defense per tier) is probably something that has to be disallowed as there are a number of "fixes" to improving defenses (various defense boosting feats, defense boosting items, etc).
