I havent had any of the 40's up and running, only the 30's and the vent is a bit thinner.
I do however have one on a 29 vert that is planted and running with no frogs but that one is intended for darts. the vent should be close to 3.5 on that one and I get some good ventilation.

now on the 30 I have my theloderma asperum in I get plenty of ventilation and its only about 1.75 inches

I would say depending what kind of mesh you are useing it will also change how much air is passed through. I use a strechy mesh with very small openings and it seem to be working fine