A 97 additional

Moreover, since "You make all pertinent decisions about a spell (range, target, area, effect, version, and so forth) when the spell comes into effect", several of which are determined by caster level (and caster level being generally irrelevant before this point), it's implied that caster level is determined when the spell comes into effect. In the case of touch spells, the target is determined by touching it, so that's when the spell has to come into effect.

(And if you got stunned any earlier than that, Reserves of Strength broadly wouldn't work on spells with a casting time of longer than 1 round since you normally wouldn't have an action to keep casting it.)