Exit Strategy to cope with RF?

[followthatcamel]

Exit Strategy to cope with RF?
Got caught out yesterday when Secret Asset got withdrawn immediately before start of 16:15 at Bev. I'd backed Fol Hollow for £200 a few seconds before off at 4.1 and put in a lay of £224.66 at 3.65 to carry into "in play" and green up for £24.66. Event went in play and lay bet got matched within about 15 seconds. The subsequent RF of 8.30% from Secret Asset meant my back odds were subsequently reduced to 3.76 whilst my lay odds stood. I originally used P&L function on BA ladder to select my original amount and price to lay at to green up for £24.66 profit. Instead I ended up losing £43.35 as horse went onto win (as always happens in these cases)!!
My question is does anyone know of a quick (rough) calculation that can be made to arrive at new amount and price to generate similar P&L after cancelling my lay bet and re-entering in play? The above all happened very quickly so I would only have had a few seconds to cancel the lay bet , perform calc and re-enter the bet, or put in another back bet to at least reduce what would be my new liability.
To my mind, the withdrawal of horses just prior to the off has happened a couple of times recently so I guess any traders who look to back front runners and lay off in running would have an exit strategy if this were to happen?
Any ideas welcome.