I don't know what is possible with betfair website as i don't use it, i use Gruss but you have 2 options, lay to payout or lay to liability.

Payout 100

Prices Stake - Loss if winner
3.00 - 33.33 - 35.56
5.00 - 20.00 - 35.56
9.00 - 11.11 - 35.56

profit if no wins 64.11 less com 60.90

Formula (payout/price)

Liability 100

Prices Stake - Loss if winner
3.00 - 50.00 - 62.50
5.00 - 25.00 - 37.50
9.00 - 12.50 - 25.00

profit if no wins 87.50 less com 83.12

Formula (payout/(price-1))