You can't achieve the impossible. If a horse hasn't raced before, as far as I'm concerned it's an unknown quantity. I don't take any notice of trials as they don't neccessarily reflect a horse's true form. I have a "what if" program which tells me how much I would have won or lost if I'd done whatever. I use it to see what my return would be if I allowed no runners without form, then I'll do a run to see what happens if I allow one new runner in a race, etc. In my old system I allow up to 50% of the field to be new runners. Slightly less PTO but compensated by the increased number of bets. 50% (for me) was a nice cut off point. Betting Quinellas I don't allow any races where there is an untried runner - I find that works better for me.
Once a horse has run it has form in my book, some may improve or get worse greatly at the second run but then many other horses aren't particularly reliable at the best of times either. Again, my what if program tells me my PTO is better if I make it a rule that every horse has to have had 6 runs but that cuts the bets down so much that the overall profit is diminished.
Just use common sense, work with what you've got and don't cry over races you can't rate, there's always another one tomorrow.

KV