Betting formula

Hi all,
I'm trying to create a spreadsheet for placing two bets - one at a bookie and one at an exchange which leave equal profit or loss.
Basiclaly - I need to fill a cell with a number which will cause my two profits at the bookies and exchange to be as close to equal as possible.
Here is an example.

Place £10 at Ladbrokes on Man U to win at odds of 2.0 meaning either £10 loss or £10 win
At Betfair I need to place a lay bet of ?? in order to cover the 5% commission and leave me with as nearer profit or loss to the bookies.
i.e if I placed a lay bet of £10 at Betfair if the bookies won I would end £0 profit/loss while if Betfair won I would end up -50p due to the comm, I need a formula which will give the lay figure to ensure both these numbers were as close as possible - in this case a lay of £10.26 would leave me with a -25p at both bookies - but how do I get the spreadsheet to work out this £10.26 figure for me?
Sorry for the long message!!