Big surprise.
I reloaded my archery party. The caster was bishop level 8 + priest level 15, with 100 divinity, 47 wizardry and 95-100 in the realms.
He is able to cast all his mage spells at level 7 in green (except x-ray, which is very light yellow at 7; mental is the realm with only 95 skill).
I also reloaded my 'music' party (3 bards, 2 gadgeteer, and a similar bishop-priest). Don't have the numbers with me, but he was at a much lower level, and was able t cast the mage spells at level 3 to 4 in green.
It looks like all 'caster' levels are used, not only levels in a class that can use the specific spell. And realm skill makes a big difference (about 10 points for one power level)
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