It's one of the preset settings--you have to go into global settings...it's not very intuitive to find it. Maybe order presets? I'll have a look when I'm on tomorrow and see if I can find it again.
What do you mean 20%? Probably your total(lifetime) PnL after tax is, for example, 800K and total win before tax is 1000K. Does this case explain your 20%? If not, please give me an example. PS) Of course this example is same as 400K (after commission and 1040 tax) and 500K too. (TaxRate=Tax/WinningBeforeTax)