I think I'm missing something here too. I realize that the addition of Ur and SC makes the calculations wonky because of their very nature, but exactly where does the loop begin?
I think I kind of see it... sort of.
Say we have
Wizard 1
Bard 1
Spellthief 1
Ur-Priest 1 (Mystic Theurge progresses it to 10)
SC 2 (MT progresses it to 10)
Mystic Theurge 10
X Arcane caster 1
So 17 class levels. Forget qualification for a sec here.
The caster levels for each would be...
SC 13 (adds an arcane spellcasting class by default; two base levels, +1 from another, then )
UP 19 (adds half all other arcane spellcasters; this includes SC, which notably increases its CL a lot thanks to MT)
Wizard 16 (Adding all those levels together)
Bard 16 (Same)
Spellthief 16 (Same)
X Arcane caster 16 (Same)
I think I see where the loop begins. Since each class would have to recalculate its CL after determining each class' CL, it gets to something like this:
SC: 29 + 16 (Wiz) + 16 (Bard) +16 (Spellthief) +16 (X) = 93 (SWEET MOTHER OF GOD!)
Wizard: 16 + 16 (Bard) + 16 (Spellthief) + 16 (X) + 29 (SC) = 93
Bard: 16 + 16 (Wiz) + 16 (Spellthief) +16 (X) + 29 (SC) = 93
X: 16 + 16 (Wiz) +16 (Spellthief) +16 (Bard) +29 (SC) = 93
Spellthief: Same as three above.
UP (Oh god...): 19 + 46.5 = 65.5
Problem is, when does the loop stop, exactly?
Also, I still don't figure how song of arcane power adds 36 to that specific build (haven't done the math in mine yet).