What is the formula to work out the back side for a Matched betting trade

Hi,

Currently I am laying and then firing off a back bet 20 ticks above my lay price , this goes in play unmatched. The issue is how do I get it to work out the amount to bet on the back bet inplay , so when it gets matched it will green across all selections.

Example:
Layed 100 at 4.20
Back 105 at 6.40
Then I have to hope my green trigger greens up before the end of race

How do I just get the first back bet at 20 ticks above my lay price to bet the amount needed to greenup if matched?

hi
use the    attempt to green up   action , it is the same as green up but it places the bet unmatched.

Thanks