Quote from Mr Subliminal:
... instead of blanketly ignoring these trades, what percentage, if any, should I allocate as executed? If this is what turns a losing system into a profitable one, is there any reason to feel inadequate or insecure?
From my experience, I came to the conclusion that it is very difficult to have any percentage of that kind.
Personnaly, I take it as 50/50.
- When I consider being executed as soon as I see a print AT my price (this is the 'optimistic view of real trading' because that would mean I am the first in the queue at that price), my system shows some decent results.
I take that as my max possible results.
- When I consider being executed as soon as I see a print UNDER my (limit buy) price (pessimistic view of real trading : i am the last in the queue at that price), my system is breaking even or has negative results.
I take that as my worst possible results.
The reality is in between. You're right, that is what makes the difference between a profitable and a losing system.
I wouldn't be confident to follow the 'optimistic' system. I prefer to find a profitable 'pessimistic' system.
If it shows some good results i know that it can't be worst in real life (psychological problems excluded

).
The only way to gauge the probability of being executed is to know where we are in the queue at that price. In my opinion, it is totally discretionary and helped by experience (bid/ask size when order sent, gut feeling about the number of front orders being pulled, tape reading,...)
That's why backtesting a scalping strategy is very difficult.
Just my 2 cents. Good luck.