Not certain that there is a formula for this, although if there is, I'm certain Corey can come up with it.
In the meantime, I think I have an answer for you, although it would involve a few steps:
1. Configure your prizes as you have described, using the percentages in your example (1st: 50%; 2nd: 30%; 3rd: 20%).
2. After buying in all of the players for the tournament and just before you start, go back to the prizes tab and change 2nd and 3rd place to a fixed amount awarded, and make it equal to the monetary value for that position (re: $75 for 2nd, $50 for 3rd). Also, now change 1st place's percentage to "100%", and the rest of the prize money will be allocated here.
From this point forward, all rebuys and add ons will go to 1st place, while 2nd and 3rd will be the fixed amount.
Kind of a roundabout way of going about it, but it works. Hope this helps.