I have been working on this for a while and you're right, it's not the most practical. I have got to the stage where I have 7 years of results for PL1/2, SWE & UK, and it has produced a list that looks broadly about right, but it'll never be perfect.
Even looking beyond the track - do some riders try more in POL than elsewhere (probably)? There so many variables (way more than I ever considered) and I don't think we'll ever get a proper, accurate model.