The relation between retention and mobile phase composition is more complex than in reversed phase HPLC. In RPLC, there is a simple relation in that the logarithm of retention factor is directly proportional to the non-polar modifier concentration:
log k' = log k'o + S.Ø
Thus, a log k' vs %B plot is linear.
In SFC this is not the case. It turns out that the relationship is a log - log plot:
log k' = log k'o + S.log Ø
This is the Soczewinski Equation, which was developed to describe normal phase chromatographic separations and which is consistent with the normal phase character of SFC chromatography. The slide show on the right gives more details.