Cheers Shaun, you're pretty much spot on.

Had $200 @ 31, and laid $363 @ 17.12.

Wins = +$6000 - (363 x 16.12) = +$148.44
Loses = -$200 + (363 x 95.45%) = +$146.48

I'm on 4.6% commission.

All I look for are runners that are 95% or less of the bookies price, and that there's plenty of liquidity. Yesterday there were none but today I got on 10 which is very unusual, and just missed a few others.