Good luck with this!
I've tried something similar over the easter weekend but took a slightly different approach:
- I restricted myself to the PHB1 and using the default ability array.
- I created one character from every class
- I wanted to have a party of five, so I doubled the striker role
- I created the characters on level 4, 14, and 24
- I picked items assuming a party had actually reached these levels by adventuring, resulting in a larger number of items than by using the DMG rules for higher level characters
- For any calculations, I used the averages of characters with identical roles.
I'm mostly okay with your reasons for doing things differently, but I'm a bit dubious about using level 21. It's the first level in the epic tier. Will this allow to come to meaningful conclusions about the epic tier? I don't think so. It's also too high to represent the paragon tier. Why choose such a threshold level?
Since your reputation on these boards is quite good, maybe you'll be able to reach a couple of the regulars who've already formed an opinion on 'the math', but I doubt it.
I definitely don't want to discourage you, though. Bring it on!