Just that i would have to use an estimate. Which i don't like doing.
Back testing is only indicative anyway i think it only gives a ballpark idea of what you can expect going forward.
In daytrading slippage can be a very important cost factor. I always used 1 tick slippage in and 1 tick slippage out. If your average profit per trade is small, your real profits can be losses in real trading. Better go for the worst case scenario in backtesting.

