Hi Mike
How about group or listed races with 15 or more starters?

My rationale is that many of these are ''grand finals'' for their participants. A big field is an indicator that there are many trainers who rate their horse a chance - i.e. unlike the small acceptances in this class of race which is an indicator that there is a vastly superior dominant horse running. Think of the field sizes raced again by such horses as Red Anchor, Kingston Town, Placid Ark etc because they were clearly dominant horses (showing my age - there are, of course, more recent examples).

Also, any open class race with a significant number of first or second up runners. Again, for the sake of value I'd limit field size to a minimum of 12 runners with 4+ first and second up. I find it difficult to bet race by race so I probably seem a bit obsessed with number rules. Retiring soon so will have the freedom and time to bet race by race and according to price.