The first question is what is the humidity (RH) in the room? if your room is at 75% the tub is going to be anyway. If your tub is higher than the room you need more ventilation, and a smaller water bowl. I use a simple rule of thumb with RH and ventilation. With paper substrate if you remove the water bowl the RH in the tub and RH in the room should be the same in one hour or less. This represents one air change per hour at the least, personally that to my mind is a minimum.