Actually, there is a passing mention in the manual - or maybe it's in one of the add-in pages - that mentions that decaf progresses from first crack to second a little more quickly. Hmmm, bet that was by that Rosetta Stone thing (item #3 in the Tips & Maintenance section). So C or the fuzzy target of C+ is easy enough, and FC or FC+, without going too far into second crack, is as tricky as ever. :-/
I've only had a few decaf roasts in the Behmor. If the quantity you expect to use isn't too small for it, I'd suggest going with an 8 oz batch to stretch everything out a little bit. P3 or P4 might help a little, but IME you're going to be on that last, full power leg when you get to the cracks anyway, so it can only help so much.
Sweet Maria's, Behmor, Encore, Bonmac, CCD, Pavina - fun AND good coffee