I should have a formula somewhere on how to calculate how much a filter pumps based on the size etc, but if I understood you right, you're wondering how much filtration a goldfish tank needs?
If so, a filter for a goldfish tank should be rated for a tank twice the size you have, OR you should have enough filtration to deal with 10 times the tank volume per hour.
Example: a 20 gallon tank needs 200gph of filtration, a 75 gallon needs 750gph.

But this doesn't sound like a calculation formula, so correct me if I'm wrong xD

Two trains of thought. 1 for mechanical filtration the higher the flow the better but 2 for bacterial filtration a slower flow giving a greater dwell time the better. One of the reasons where sponge filters along with HOB or canisters would seem to be the tactics for superior filtration for goldfish. I've been using  a Fluval C series HOB  but just ordered a Hydro Sponge to determine if my theory has merit.