We've done this before. I think someone worked out that it goes something like this:

R1: 32%
R2: 31%
R3: 31%
R4: 30%
R5: 30%
R6: 29%
R7: 29%
R8: 28%

Might even have been a slightly bigger difference from R1 to R8, but I think that's fairly close.