If I did a water change every 2 days, what percentage of water would I need to change every 2 days if I was to give the fish the same benefit as a 50% water change once a week?

I'm assuming it's not necessarily a case of simple maths? Or is it? Because in that case, it would be about 14% water change every 2 days (which somehow doesn't seem enough, even though the maths check out).