is it fair to assume a horses class by the prizemoney won in calculating a system formula. if not how do others judge class?

You can use prizemoney as a guide only, certain adjustments need to be made for class as well, as some maidens are worth $4,000 and some are worth $40,000 as an example.

I would add a score for the track to the prizemoney and even a score for the average prizemoney of all competitors in the race it contested.

Just some ideas.