So Cary, did you see this in the Fantastic Vent instruction manual? Sounds like you can adjust how much the vent is open. This is from the 6000 RBTA manual, not sure which one you are now offering. Similar instruction for the 6000R/417.
Dome adjustment or emergency close knob. To stop dome partially open; apply
opposite force to slowly rotating knob, as dome is OPENING ONLY!! To adjust
dome, allow it to travel all the way up automatically. Now pull knob down to
“MANUAL” position. Turn knob lowering dome to desired height, them immediately
push the knob back to “AUTO” position. (NOTE: at 1/3 open, fan blade exhaust
efficiency is reduced to 90%). After adjusting, always check knob to ensure it is now
“locked into gearbox”