Actually I wasn't trying to balance out a party just using some shitty math, I will deal with that on my own. I'm just trying to calculate the way a party functions so that I can hand out encounters of appropriate difficulty.
If a 10th level wizard is a CR 14, then he'd be a fair challenge, by the ad-hoc system jameswilliamoogle just posted, for a party of 4 martial adepts, level 10, because their tier is 3, meaning (level x 1.0) + 2 for each doubling of the number of monsters. A party of 4 martial adepts, level 10, and a single wizard, level 10, sound about matched under many conditions, assuming the wizard's using regular PC wealth to buy disposable stuff and using BfC spells and Polymorph, like a good little tier 1 wizard. Iron Heart Surge, with its OP goodness, could throw a monkey wrench into this, though.
This kinda might break down a bit, though, because a party of 4 wizards, level 10, is a CR 18, but a level 13 wizard is not really an appropriate challenge for these 4 wizards. This is due to action economy breakdown as well as rapidly scaling CR increases for straight multipliers. Maybe a system can be made where a character is balanced against a CR, but this one's a tiny bit off.
New CR system, anyone?