Radon Calculations - Health Physics Question

I rigged up a very basic radon test in my basement - a MERV 11 20"x20"x1" filter attached to a box fan running on low speed over one week. I believe this is enough time for there to reach an equilibrium level of daughter products on the filter, though I may be wrong. I measured the filter w/ a GM detector (Ludlum Model 26) yesterday and found a pretty even distribution of ~1000cpm over the area of the filter on contact. I then put a piece of paper b/w the filter and the GM, which reduced counts to ~500cpm. Playing with these numbers, I'm trying to see if there's a way to (very) roughly equate this to a concentration to compare to the 4 pCi/L remediation limit. As best I can tell, I need the efficiency of the GM, the volume of air moved through the filter, and some radon daughter product equilibrium information, which is the one I'm struggling with. The fan specs are 1450 CFM on low, but the filter reduces this value dramatically. Is this a solvable problem? I know the uncertainty will be huge, so I will be getting professional testing, I'm just curious from a theoretical POV and also to see some potential ballpark figures.