How can I calculate the performance if I deduct 0.42% fee on a monthly basis?

[boyac]

How can I calculate the performance if I deduct 0.18% fee on a monthly basis? And how can conditioning to further deducting fees if the portfolio price pass a certain level, for say 105 then I deduct 0.42%.

Instead of a fixed amount, the monthly fee deduction is based on the total amount of the portfolio.
Should I change
bt.algos.CapitalFlow(), or bt.algos.Rebalance()?

thanks