I have been successfully laying the field with S.P. using a different system which is based on Betfair's prices.

Today there were four races that were "all green". Out of interest, in the fourth race I calculated the S.P. market. My calcs came to a 100% market, as would be expected.

I could easily accept my calcs were incorrect, but because the worst possible result was more than a 6% POT (based on the highest liability layed on any horse in the race) I would have had to badly miscalculate the value of the market. There were no late scratchings in any of the four races.

If all the four races did actually have a 100% market it would be mathematically impossible to have an "all green" race, seeing that the price does not change.

Any ideas?