Greening up (mathematician needed)

[ebogga]

After backing Grasp in the last race at Kempton tonight I (once again) started wondering about greening up i.e. making a guaranteed profit by laying lower than the backing price. I was on Grasp at 7.8 and it was available to lay at 5.0 before the race. That´s a 56% difference in the prices, would´ve guaranteed a nice profit not depending on the outcome. I´m not that advanced in the mathematics behind betting, I could be WAY off with this but hopefully someone will correct me. When I backed the horse at 7.8 that implies a 12.8% probability of the horse winning. Now, if the price shortens and is available to lay at 12.8%+5% (commission) = 17.8% lower, should I green up every time? Or should I compare the probability implied by the lowered price, in this case 5.0 -> 20%. Another idea would be to use the strike rate of my bets as the limit for greening up? I.e. if I had a 15% strike rate, price shortening beyond that means it would make sense to green up. The mathematics behind this really baffle me, help please! :unsure I always seem to get locked into thinking that "they´re backing it off the boards, it´ll surely win now" and never take the guaranteed profit. Would be nice to have some mathematics behind to help making the decision. ;)

[mowgli77]

Re: Greening up (mathematician needed) I think Slapdash or maybe Ginge will be your best men for this, if you don't get any replies try sending them a PM of this post. I'd love to help but maths isn't my strongest point either. :$