How this calculator works
The fee is modelled the way a fund's TER actually works: it is deducted from the return every year, so the "after fees" portfolio compounds at gross return minus fee. The gap between the two lines widens dramatically over time, because the fee does not just cost you the money taken — it costs you everything that money would have earned.
This is why the difference between a 0.2% index fund and a 1.5% active fund is not 1.3%; over thirty years it can be a third of your final portfolio.