From my (limited) experience it seems to be about double P76. And I have heard many say 500+ selections is there benchmark as far as being comfortable it may continue to perform. Think youre on the money there but in saying that I wouldn't drop something promising purely because the number of bets isn't there.

To me there is 2 types of mechanical systems - Longshot (Lower SR but potentailly higher POT) and High Strike Rate (high POT unlikely). You can't have both over a decent period despite what you might get sent in the mail....

The better longshot systems I've come accross have around 20-25% SR for the win and 50% SR for the place. So your backing something that is usually at a decent price (each way) and half the time your getting a return. Most I've spoken to use these types of systems as bit of fun, small bets, as a means to have enough bets for interest and to stop from chasing more bets .... having bigger bank/bets on the higher SR systems.

Of those systems the better ones I've come across tend to be mid to high 30's% SR and around 65 - 70% place SR. So again about double. All sounds nice but there is still decent runs of outs that test the metal. If you have a system of this ilk you can try running place doubles or trebles but it takes more time and effort on a Saturday afternoon and although the outs may be less so might the return.

You need to find something that suits you personally. As I've stated in other threads I'm not super keen on purely mechanical systems at present, I think they have value but IMHO you need to incorporate speed ratings or time ratings or pricing brackets or some type of form study/handicapping however minor.

Backfitting is a trap but there is no other way to 'improve' a system but it can be done logically or foolishly. I think you need to ask yourself questions when you trim a criteria or bring in a new rule not just say it must be good cause the figures improve.

Could dribble on for hours so better pull up but happy to give my opinion and account of some of my experiences anytime.

Grub