If you're looking for something rough, I think a linear conversion would work. With x264, lower numbers mean higher quality so it would look something like this:
Quality = MaxQuality - QualityFraction * (MaxQuality - MinQuality)
So if you had a quality fraction of 0.55, it would convert like so:
51 - 0.55 * (51 - 0) = 22.5
I believe that HandBrake is also moving away from using the fractions/percents and going with the direct quality targets.