Maths experts - help required

On average, day in day out I average 10 winners out of 30, prices vary from odds on to 20s.

Divided into legs of 10 races I average 3-4 each leg.

The spread each leg is
2 to 6 out of 10 for the first 10.
3 to 6 out of 10 for the second 10.
0 to 7 out of 10 for the third 10.

Is there a formula for me to combine 8/9/10 without obviously targetting 3/4 out of 10 each leg or 8/9/10 out of 30 as the cost would obviously be prohibitive?

Thanks in advance.