If your not collecting betfair prices that you could lay to in order to test laying systems then you are going to lose.

If your using tab prices then the only way you can assume the results are about right with a 15% increase in odds is in the < $5 range. After that you want to add 30% up to $10 and then 100% up to $50 and then 500% for anything longer. Even then it might be higher.