A fit question for those of you with a bike fit you really like on both your road bike and your MTB -- is your MTB top tube longer/shorter (and how much?) or the same as your road bike?

Should MTB top tube length = road bike top tube legth plus the extra reach between the road bars and brake hoods (assuming the same stem length)?

I really like my road bike position, but have not yet found an equally good fit position on the mountain bike (same 120mm stem length on both), and was wondering if anyone else has a perspective or rule of thumb on top tube length differences. Thanks.