Might consider a gift certificate to a bike shop instead. You can ask a million different people and the best you'll do is narrow it down to 2 or 3 possible sizes.

I really don't understand why road bikes are still sized by their seat tube... The top tube length, being just about the only thing you can't adjust, is also the important one.

pcad: can you post a picture of your MTB with the 400mm stem? I think that'd be pretty amusing to look at..