Lay question

[homero]

If i back $2 at say 10 and then lay $2 at 9, i wither win the back or get my stake with the lay.

If i greenup, i get 50% of profit either win or lose.

How much do i have to lay, in order to profit from the lay and get my stake from the back??

I found a calculator online with a slider that gives me the amount, but i would like to know the math behind.

For example, yesterday at 5.30 southwell i made my best trade EVER!
I got Ptolomeos at 120 and close the trade two ticks before reversal.

But instead of greening up, i would like to profit from the lay and to cover with the back

[Wednesday 13]

How much do i have to lay, in order to profit from the lay and get my stake from the back??

To accomplish that, profit from back bet should be equal to liability in lay bet.

Your profit from back bet is:

(Back Odds - 1) * Back Stake

Liability in your lay bet is:

(Lay Odds - 1) * Lay Stake

So:

Lay Stake = (Back Odds - 1) * Back Stake / (Lay Odds - 1).

In your example:

Lay Stake = (10 - 1) * 2 / (9 - 1) = 2.25.

If your selection wins, you win 18 units from back bet and lose (9-1)*2.25 = 18 units in lay bet, so you break even. If your selection does not win, you win 2.25 units from your lay bet, and lose 2 units of your back bet, for profit of 0.25 units before commission.

[homero]

Thanks a lot man!

[doubleback]

I found a calculator online with a slider that gives me the amount, but i would like to know the math behind.

I assume that you are talking about the Chromaweb Calculator. I knocked-up an Excel version and posted the formulas in this fred, a while back.

[homero]

Thanks man, always usefull