Here's a couple tables I made, because I was bored:
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{table]|1 PrC at a time / LA+HD as classes|any # of PrCs at once/ LA+HD as classes|1 PrC at a time / LA+HD on all sides as once| any # of PrCs/ LA+HD on all sides at once
Non-gestalt|x1|x1|x1|x1
"double"/standard Gestalt|x1.5|x2|x1.25|x1.75
"triple" gestalt|x2|x3|x1.5|x2.5[/table]
{table]|Dual Progression PrCs|No DPPrCs, Progress + Base|No DPPrCs, No progression w/ base
Non-gestalt|x1|x1|x1
Standard Gestalt|x1.5|x1.25|x1
"Triple" Gestalt|x2|x1.5|x1[/table]
Multiply the character's level by the value in the first table to determine their "effective" level for calculating Xp, or divide the enemy's CR by the same value instead. not both.
If you want to deal with the second table, add the two multipliers from each table, following normal rules for adding multipliers ((x2)+(x2)=(x3)).
By 1 PrC at a time, I mean you can't advance more than one PrC at any given level. And by any # of PrCs, I of course mean you can take a level of three different PrCs, so long as you qualify, on the same level.
By LA+HD as classes, I mean you can put LA and HD in your progression the same way as classes. So, LA 2/class 18//HD 4/class 16. By LA+HD on all sides, I mean that LA affects all sides of the gestalt, and same with racial HD. So, LA 2/HD 4/class 14//LA/HD 4/class 14.
By Dual Progression PrCs, I mean, sure, you can take mystic theurge. Take it at the same time as continuing levels of wizard, if you want. By No DPPrCs, Progress + Base, you can't take mystic theurge, or other dual progression PrCs, but you can take a wizard PrC at the same time as wizard. And by No DPPrCs, no Progression w/ Base, you need to take a different base class with your wizard PrC. By extension, if you can take more than 1 PrC per level, no taking more than one PrC that advances the same class at once.
It's probably best you DON'T use my tables, and that you DON'T use the second table if you use the first anyways.
Also:
Archivist 3/Mystic Theurge10/Archivist 4-10//Wizard 12/Archmage 5/wizard 13-15//psion 3/Cerebremancer 10/psion 4-10
Hey, look at that. Effectively wizard level 40. SAD. Minimum level Archmage entry. And how is it wizard 40? 15 wizard levels (15), 5 archmage (20), 10 Mystic theurge (30), 10 cerebremancer (40). AND it's effectively archivist 20 and psion 20. No Capstones, but meh.
All it's missing is factotum.