Calculating Shift Loadings

I need a bit of a help here. What I'm looking for is basically a formula to calculate shift loading differences. For ex.: Total hrs worked 86. As a part timer, my ordinary hrs are 76. Whatever is over 76 comes as double time ("F").

A B C D E F
87 76 42.5 15 8 11

Thats how it looks initially. "F" is the double time hrs paid (the difference between Ordinary hrs and total hrs worked). Which means that I need a formula to automatically calculate my 20% ("C"), 50% ("D") and 100% ("E") loadings. Correct would be like this: A=87, B=76, C=42.5, D12 and E=0. So, basically this 11 hrs straight double time has to come off the 100% ("E") first and then move on to 50% ("D") and 20% ("C") if necessary. So, need a formula to fields C, D and E.