prenup question regarding property

hello,

my fiancee and i are considering a prenup, and one of the key issues is regarding her property. she bought the apartment we are now living in few years back before we met, and it has since doubled in value. now that we are getting married, we have decided to split the monthly mortgage payment in half. in the process i hope to gain equity of the place proportional to my payments.

what i am wondering is when we move in a couple of years and decide to sell this place, what would be my "fair" share? is it common practice to divide the sale proceeds proportional to the equity we each own of the place? if so, should equity be considered as just the "principle" portion of the mortgage, or as the total amount of mortgage paid?

how about the value appreciation of the place before i moved in - is it acceptable for her to keep all the appreciation up to the point of our marriage, then split any future potential increase according to equity owned?

this is probably not in any case a unique situation, with once spouse having property vs none for the other. i hope to gain some insight here to see what has worked for other people or the common practice. any advise/comments are welcomed.

much thanks in advance!
skippy

You often can agree to share appreciation going forward, but not for what has already occurred.