Usually, R/C models have bigger tailfins than their full-size counterparts so additional fin area is not needed. Scale Cubs have a ventral fin. Some scale beavers have finlets on the Horiz stab (a Kenmore Air modification. Also, R/C models usually have less dihedral than full-scale planes, so the vert stab is not outvoted by the vertical component of the wing area, so they're not as likely to get into "dutch roll".

I'm sure there is a method of calculating it, but not at my disposal.