It is normally not recommended to have variable pipcost in a strategy test, but if you still want it, a function for this is in the manual under "PIPCost".
Why is not reccomended?
For a backtest from 2010 to 2018 on EURUSD for example the pipCost change too much in the perdio considered.
With PipCost equal within backtest period is not right. Are you agree with me?
