How much ventilation is required for a tank

I adopted a Ball Python a few months ago. She came in a 40 gal tank with a screen top, so needless to say I've been struggling with humidity (temp's are good with the CHE and two UTH all on thermostats). I tried the tin foil on the top, but did not get the results I wanted. I have since custom cut lexan/plexiglass to fit the screen with a cut out for the CHE and 8 1/2 inch holes in the cold side. There is also a 4x4in door in the screen that I have another piece covering.

My question is how much ventilation is required to keep her happy? The CHE hole is about 1/2 in larger than the CHE housing, will that and 8 1/2 in holes on the other side be enough, or should I remove the 4x4 piece over the door?

At some point I will need to get a proper enclosure, but for now the tank will have to due, so I'm trying to make it work...

Should be fine. I personally use a thick piece of cardboard cut to fit the top of the screen. Then I cut a circle that is 1" bigger than the dome for the CHE and that's all I use. Gives plenty of air. Of course my carboard/tinfoil top isn't airtight either. The plexi top should be ok. If you are worried though, you can remove the cover on the door or drill a few holes in the plexi. They really don't need a huge air hole. Look at all the PVC cages, they usually just have a few slits for air holes and they work fine.