Pauls123,

I don't know if this is helpful, but to calculate the return on S.P. laying for a liability of $30 is:

1) deduct $1 from the S.P price.
2) then divide $30 by it.

For example, if the S.P. price is $4.40, we deduct $1, leaving $3.30.

We then divide $30 by 3.30 = $9.09 (this does not include Betfair's commission).

Contrary to backing, with laying and there is a late scratching, this does not reduce the S.P. price, it reduces the liability.