people will tell you you need an expensive variac.....i've used one of these for four years....works perfect...i leave it set to about 20%, flip it on full when the lights come on, flip it to variable when they go off, works like a charm.
this is for fans that have brush motors, you may need a variac if you have a fan with a brushless motor, but those are typically very expensive, and come with built in controllers for the most part
https://www.amazon.com/Yescom-Variable-Controller-Settings-Hydroponics/dp/B00U3LD1WA/ref=sr_1_1_sspa?ie=UTF8&qid=1548260811&sr=8-1-spons&keywords=router+speed+controller&psc=1
just open your bottom vents a little, turn your fan on, and dial it up till you start getting a little negative pressure, then leave it there. i leave mine running full for an hour or so after the lights go off, to prevent a big RH spike, then i turn it down to about 20%, kick it back up to full when the lights come back on.