One thing I have found useful is to preserve the distance from the center of the saddle to the primary hand position (for me, hoods on the road bike, and where the brakes are on the mountain bike). Many folks prefer to ride more upright on a mountain bike, so in a sense this gives an upper bound. Stems tend to be much shorter on mountain bikes, but the hand position is further out (depending upon flat bar width, it might be greatly extended).