I actually like the incapacitation rules quite a bit, since I like that it leaves room for powerful spells without completely wrecking important encounters. However, there is one problem I have: Incapacitation spells are better at odd levels than even levels. For example, casting a 4th level incap spell at 7th level allows you to affect creatures one level higher than you normally. However, at 8th level, you can no longer affect creatures one level higher than you normally with the same 4th level spell. It feels weird from a narrative sense and feels bad from a mechanical one, where incap spells are more effective at some levels than others. I've been trying to think of a solution without completely changing the incapacitation system, but I'm stumped. Any ideas?
If you don't mind making them slightly weaker, you could fix it with "Incapacitation spells work on creatures up to double the spell's level, or your character level, whichever is lower".
And if you want to round up instead of down, "Incapacitation spells work on creatures up to (double the spell's level) plus 1, or your character level +1, whichever is lower."
I'm afraid the "whichever is lower" is going to feel like it penalizes people at odd levels, though.
Hmmm, maybe use the official Incapacitation rule, but add "If your character level is an even number, add 1 to the max level it can affect." Same result as my round-up version, but without people having to take the lower so it would feel better in play. Although then there's a weird oscillation of the levels that lower-level slots can affect, and that's no good. Sigh.
Final try: "If your character level exactly equals twice the spell level, add 1 to the max level it can affect."
I like your final solution Sintra, that'll probably be the one I go with.