I started making out a formula to do what you wanted but it was getting really confusing and in the end it doesn't do anything special. The bottom line, you have to assign prizes in the prize tab, and once you do that you have your buy in and your winnings done and that is exactly what you asked for.
Another reason its not good to make a big long formula is that if in the future you decide to change the winnings or the buy in, then you have to go through the formula and manually change it, and if you aren't good at formulas, that can be a major headache. With the tw + tc formula then it doesn't matter what your buy in is or what your winnings are, the points will come out accordingly.
I just can't see a way around the prize schedule. Its fine and dandy if you just do the prizes without using the program, but you still have to figure that out. One extra step of putting whatever you decide on into the prize tab then everything else works itself out for you, using the tc + tw formula.