CFM Calculation with Speed Controller?

The $20 fan speed controllers all have Low, Medium, and High settings. Any idea how to estimate how these effect CFM? If there's a formula, great, but I suspect there isn't, and I have no idea how to guess.

In particular, I'm considering a 4" 190 CFM inline fan. Run it through a carbon filter, and that brings the CFM down -- I've read 25%, does that sound about right? There will be a single 90 degree turn in the venting, which brings it down again -- any estimates of how much? And then the big question, put it on a speed controller and set it to Medium or Low, what happens to the CFM then?

My tent is 2x4x7 = 56 cubic feet and lit with LEDs; I'm looking for a complete air change every 2 or 3 minutes to provide CO2 without losing more heat or humidity (growing in soil) than I have to, so I need to be between 19 and 28 CFM. My gut is that the 190 CFM fan will just be too big, and as that is more or less the smallest inline fan available, I'll need a different solution entirely. Thanks for any advice.

CFM x 60 = 11,400 / 56 = 203.5 ach

If your exhaust is running through the carbon filter at that speed the carbon isn't doing anything to mitigate odor. Activated carbon needs 'dwell time" to be effective. I would cut the controller back at least 50% - you will have roughly 100 ach and less odor.

They are usually still graduated. All the way up is 100%, all the way down is usually around 25%, just past center to the high side is 50%. Max fan speed x %=actual fan speed.

I think a 4"inline @50% might be to much. I'd say duct fan, but I don't think they'll pull through a filter.
Is stealth important? If not, put the fan outside the tent, mount a "T" inline, put a damper on the "T" outlet, use that to modulate the fans vacuum on the tent. Between that and the speed controller you should be able to dial it down.

Thanks -- I'm thinking it really is too powerful, and I shouldn't have to fight it so. The other possibility is a homemade system using a 50 CFM bathroom exhaust fan, as described here: https://www.icmag.com/ic/showthread.php?t=45196.
Stealth is somewhat important -- about to be legal in my state, but still illegal in the country, and who knows what the deal will be after the power shifts hands. Quiet is also important, and I suspect the bathroom fan will be a lot quieter than the inline.

WeedFreak78: Took me a full day to realize the true potential of your suggestion. Here's a modification: I could leave the fan inside the tent (on a speed controller, dialed down), and split the output in two -- half exhausts out of the tent, the other half just vents back into the tent. Helps with air circulation, and ought to divide the throughput of the fan right in half (probably at the expense of some extra noise).

Gonna mull that one for a while (opinions welcome, of course).