laying at sp

Hi, can you tell me how to configure my trigger to lay an amount ( percentage of bank at sp )The options on the lay at sp  trigger condition allow me to set my price limit at at sp but then force me to use liability rather than amount for the bet size.

The reason you are "forced" to specify the liability lies within the rules of SP betting. BetFair can guarantee that it will match your bet to the fixed liability, but the amount cannot be predicted as the actual price at which it'll be matched is unknown until the market is off.