Greening across selected runners ?

[lampac34]

Does anyone know the formula for greening/redding across multiple runners all with different profit and loss results ?

eg: 10 runner race:
Runner 1 = - 5
Runner 2 = - 8
Runner 3 = + 12
Runner 4 = + 27
Runner 5 = - 8
Runner 6 = -1.85
Runner 7 = - 1.85

So from the above example I would want to green up across all selections, and know how to green up so that say the top 3 only are in profit and the rest of he field is a £0.

Is this possible ?

I have a system that sometimes shows a full green field, but other times the winner shows a loss, so if I can green up across the lowest profit runners in the last few stages to a maximum risk sum then I can break even on some races and have a full green on others.

If anyone knows the formula or if you know or a resource/location on line where the maths are explained would be most grateful.

Many thanks

Tim