How to include fees in backtests

Rule of thumb for applying fees to backtests? If a flat % then what would be a conservative value? If by other means then please elaborate.

All depends on the broker where you have a live account, or you want to trade. If you have an account in Market Maker you will have different fees than ECN. Regards Greg

Would I be right in saying average spread/price will give a good estimate of fee %?

This broker has spread as fee, so you can increase spread in backtest.