13-56, I have that on 24 5/8s and 25.5 scale guitars in C and B. Works fine for me, can even do dropped A# with reasonable tension but if you want it TIGHT and to avoid "boing" you might want to go heavier.
I've tried experimenting with some lower tunings on my PRS custom 22 which is 25" scale, when I was tuning to drop B I used the D'addario EXL-148 12-60 Extra Heavy set. I like my strings on the tighter side usually But the problem I ran into was the Low B string and the F# string sounded too muddy when played together when distorted and I couldn't achieve clarity with the Low B string. They sounded amazing on a clean channel tho and I came to the conclusion that the problem had to be either with the PRS Dragon II pickups not being able to handle the lower tunings or the fact that I have one of those PRS stoptails WITHOUT the individual string saddles for intonation, I'm guessing it was a combination of both.
i can pull off a stable drop b on my 24 5/8 inch scale St3 with 11-48 but as far as feel goes anything less than 11s in standard or 12s in D feels plain wrong to me.