That’s why they are fixed percentage not fixed. Trading fees go up with the amount of capital moved but mostly in a linear fashion.

Calculating your trading costs (and usually more importantly slippage) is absolutely table stakes for a fund that trades.

For most funds that’s relatively easy as the trading component is a cost center that you can outsource for predictable prices.

For funds that aren’t treating trades as cost centers, well it’s presumably part of what you are selling so you better be good at it.