Hi Darky.
This technique of laying is probably the safest to use.

Divide the runners price into a payout figure.

Example.
Payout fig. say $30
Price say 6.00
30/6 = O/L $5.00

If we get bit = we lose 5.00 x 5/1 (6.00) = 25.00
If it falls over for us - we win $5.00

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Cheers.