As noted earlier, no personal expertise here, but how about thinking about it this way?

Your compressor took your 60 gallon receiver from 80 PSI to 100 PSI in 80 seconds. As noted earlier in the thread, 60 gallons is 8 cubic feet.

The air that was added to the receiver during that time occupied 8 cubic feet at 20 PSI. How much volume did that air occupy when it was at atmospheric pressure (call that 14.7 PSI) when it was "free air" at the inlet?

8 cubic feet x 20 / 14.7 = 10.9 cubic feet at atmospheric pressure (Yes, I'm pretending that Boyle's law is good enough here. Yes, I'm ignoring any change in temperature. Yes, I'm ignoring condensation of water.)

How long did it take your compressor to do this? 80 seconds is 1.33 minutes.

10.9 cubic feet at atmospheric pressure / 1.33 minutes = 8.2 cubic feet of air at atmospheric pressure per minute. (That's what I understand "free air delivery" to mean; someone please step in here and correct me if that's wrong.)

A similar 20 PSI rise from 40 to 60 PSI required only one minute. Applying the same sort of thinking there would show 10.9 CFM when working in that pressure range.

A 10 PSI rise from 100 to 110 PSI required 50 seconds. Applying the same sort of thinking would show 6.5 CFM in that pressure range.