Shaun, I think I can work out the maths behind this if you'd like, just need some clarification on it.

Is what your saying the following:

Lay 1 runner to liability of $100.

i.e. in your example if the $4.38 horse wins, you payout $100 however if it loses you receive $29.58 ($100/(4.38-1))

However you also want to lay 2 other runners as 'savers' which I take to mean you want the fact that if your $4.38 horse wins, you still break even from the lay's on the other two horses, is this correct?

I'll work on it now based on the above as I assume thats what you mean.