Flawed a little but helpful. I would estimate the charge time calculation to apply to the lower pressure as the useful cfm output. It would be biased slightly in your favour. Sounds to me like about a two horsepower single stage setup in decent shape.

My large v twin compressor is tired now and I will have to rebuild the head to make it really useful anymore.

I have a spare modern vertical tank compressor as well that is junk compared to the past compressors. Head runs at too great a speed and the horspower rating is deceptive. They call it a five horse I think but at best it is a three horse. I would never work it hard or it would throw in the towel.

At least the head is cast iron. That may be the only good thing.The aluminium heads seem to crack etc.