Do you have any guidelines you use for ventilation/airflow?
No, only because temperatures in the tank are usually going to be more important. So, ventilation is usually considered adequate if the temperatures are proper. If you have condensation building up on your tank walls you have a couple issues
A normal tank with a mesh top is going to have plenty of ventilation. A DIY enclosure is typically going to be equal to better in ventilation even with smaller vent holes due to placement of them. A convection will
occur, and is usually what is attempted by placing vents low and high.
I have three in my enclosure and they all read pretty consistently with each other.
It would be easy to test, you'd really only need to test one of your hygrometers if the other two agree. I've posted the "Salt Test" a few times over the years for testing hygrometers. If you're unfamiliar with it I can post a guide or a link to another guide on it.
-Brandon