It's pretty basic. The percentage starts at 99% (because you're not guaranteed love by being a healthy BMI) and then simply reduces by 6.5% from the previous marker for each BMI point (i.e. it's not a linear reduction of 6.5 - 6.5 - 6.5, but 93.5% of 99 = X -> 93.5% of X = Y -> 93.5% of Y = Z) because there is always a theoretical chance that fats, no matter how large, find someone, as is unfortunately evidenced in reality.