Vapour-pressure deficit - Wikipedia.

It's not really practical to worry about until you have sensors watching the readings and activating pumps, which is what I'm working on.

Have you looked into it?

No.

As relative humidity is the ratio between the amount of moisture in the air versus the maximum it can hold at that temperature - i.e., more or less the same thing - and can be managed simply using humidistat-controlled foggers (if you need that precise control in the first place), why bother?

Don't get me wrong, as an engineer, I LOVE having control over the environment, but there is a limit.