My 20" exhibition prints are single matted (4-ply) and t-hinged to archival foam core board with a 1/4" reveal and 3" mat. The frames are 26-1/2" on the long side.
For me, the #11 profile (matte black) works best both in terms of the rabbet size and overall frame depth. Also, I prefer the flat top frames. It does get a bit tight with the spring clips, though, so I have to open them up a bit before installing.
Since your frames are larger (or you're double matting) , the #15 frames may be a better choice as they're almost 1/4" deeper and might look a bit better.