hedging a LAY SP BET with BACK SP BET

Hi Guys
 
Can you help me with a trigger expression for “back at sp” bet relative to a “lay at sp liability” bet?
 
For instance, lay for a liability of £20 on a selection that turns in play to a bsp of 5.4 so the stake is £4.54
 I already have a hedge back bet at sp waiting to turn into play .
 
I want to place a “back at sp” hedge for 10% less than the lay stake (£4.09)
 
I tried the trigger expression lay_sptotal/(actual_sp*0.9) but that fails to work as the logs have the expression of “actual sp” as “0” (I was under the impression that when the race turns “in play” the sp is available for the trigger to work but clearly not). 

So this is the expression I adopted which comes close, but if the selection is drifting or steaming the hedge can be way out of kilter   lay_sptotal/back_price*0.9
 
Any help appreciated as always guys ?

Hi
The problem is a "back at sp" action and it's related back amount will only be accepted by Betfair when the market is idle, i.e. when actual_sp is zero.

I think near_sp or far_sp is better than back_price for your expression (or you could derive an average of the three), as close to the off as possible e.g. race_status is between 4 and 6 

Hi Mark 
You are a hero as always and thanks for the reply!
Just to confirm that i already have the hedge set at less than 0 to start of a greyhound race and the lay bets and backs bet are only 3 seconds apart, I have also tried to use backpcdif over 0.5 of a min to sperate steamers and drifter before the off but even this is failing, so I may have to try the idea you have suggested. 
Can you help me with the trigger expression please to average out all three prices to calculate an "approx" back bet stake. It should leave a small liability on the original lay of around £1 to a £10 lay at sp 
Thanks in advance Mark GREAT JOB YOU ALL DO!
Happy New Year 
 

Hi bfw
If you think about it, it's trying to guess BSP beforehand lol, not an exact science.

Didn't realise its greyhounds, so liquidity will be much less than horses and prices will be more volatile. Also the race status I mentioned would not apply. Having looked at some greyhound prices as they get nearer to the off, I would suggest only using near_sp and perhaps back_price or last_traded.

multiple price average expression examples:
lay_sptotal/((near_sp+last_traded)/2)*0.9
lay_sptotal/((near_sp+last_traded+back_price)/3)*0.9

I think you should try these out and see what gets closest to actual_sp

Thanks a lot Mark i will give those a try and see what happens in test 
Thanks again for your time and your help !