How to Lay at SP - with a fixed amount

In horse racing I wish to lay at SP at a known stake.

I realise that in selecting 'Lay at SP' in MF Pro one puts the maximum liability in the Amount box, rather than the staked amount.

But...does anyone know how to lay at SP for a specified amount?

Thanks.

How it could be done on the BetFair site?
Nobody knows what will be the real SP price. So you can set the liability only.

Thanks for the reply.
Ideally, I would like MF Pro to do the following:

1. check the current back_price 0.1 min before off
2. If < [10], then place a lay bet at £[2.50] at SP
3. that's it

Do you think this is feasbile?