Ah yes, the mysterious WF. I don't think there is a formula. After several years of using QL, I think I am finally using WF properly which is to say I change it liberally to get the results to agree with measured or published data - especially where the pressure is known.
I used to think that WF was fixed for a given cartridge but it isn't. If you think about it, it makes sense that it changes even by adding a few grains of powder as it represents the amount of powder sent out of the case to burn in the bore.