Let's say a good MF lens can give 80 lpmm. Since a 645 film window is 1.5 times as long as a 35mm window, the 35mm lens (Leica or whatever) would have to give 1.5 * 80 lpmm to give the same resolution on a same-size print. That's 120 lpmm.

Assuming ideal condtions and the same film type in both cameras, I think 80 lpmm in MF is far more realistic than 120 lpmm - even in a Leica.