Thanks CP
Impact Values is the term I was looking for.
I'll do some more research as Beton suggests.

Thanks also AngryP
I'll have a read of that and see if it gives me something.

Back to trying to put a figure on "Class", I thought Shaun made a pretty good fist of it in his early "New Ratings" spreadsheet where he used prize money change and beaten margins to give a class figure.