If the bell is directly in the current of the outflow of a filter return, The dissolution should be increased no matter how high it is. The idea is to keep as much CO2 in the bell till it can desolve. Current flowing directly over and around the bell will increase efficiency. Also , limit your surface disturbance if possible to minimize CO2 loss through the tanks surface.
Surface disturbace will. like aeration. current over a pure CO2 bubble will draw more CO2 from the bubble. It is like the reverse of the drying process. You dry your hair a bit quicker with a blow dryer right? That is because of the air currentpassing quickly and drawing the evaporation from your wet hair.
(I am a simple person that example makes sense to me)
Here is another and probably more relative example.
Fish breath in water. The reason is to expel CO2 and Intake O2. It is necessary for the water to be moving over the gills for the fish to breath ,right?
Let the water move over the CO2 bubble so that it can breath. And dissolve faster into the water of the aquarium.
If the surface of the tank has alot of disturbace and rapid current. In the form of waves or whatever, the tank is breathing more with the air to the rest of the room. It loses more CO2 to the room if this happens.