I don't yet know a better way to do this than using a threshold!

I think if you assume perf is normally distributed, you can still get some of the math to work out. But I will need to think more about this... if I ever choose this adventure, I'll post an update on https://github.com/hauntsaninja/git_bayesect/issues/25

(I really enjoy how many generalisations there are of this problem :-) )