What is the ideal gallon ratio from the main display tank, to your sump tank size, that you like to keep? Is there any specific reason that you use this ratio (ex; to have enough buffer room water parameter-wise, in case of some evaporation, etc.)?
I think I've seen people recommend personally around a 25-50% ratio, so if your main tank is a 100-gallon setup, then you would want somewhere between a 20-gallon or 50-gallon sump.