Chances of bet getting matched according to BSP

[jupiter_man]

I am working on a horse racing strategy that places a back / lay order according to the BSP (lets say between 2 to 3)
So basically I need to set the rule as:
at 00:00(Inplay), if 2 <= BSP <= 3, then place a £100 back / lay order at BSP.

As it is very often to see some crazy movement at the off (horse had a great / poor start). so what is the chances that it will get matched?
Or shall I use the average traded price of last 15secs as an indicator, then place a (1.01 / 1000)bet and set 'take SP'? And cancel it if the odds is out of range?

Also I will be running this on excel, will it be much slower than BetAngel's automation?


p.s. I will be running this on the BetAngel's VPS so internet speed is not a concern

Thanks for your help guys!!