I did not get into this problem as I thought some information might be lacking. As soon as I started to break the problem down that is.

The right way to calculate air delivery perhaps is by speed of the compressor including its cylinderical volume. If two stage will make a differance as well.

I was thinking about 6-8 cubic feet a minute as a general range though at say ninety pounds pressure. That may be too optimistic though. Some comparison with simular setups by compressor manufactureres might give a better guideline.

Losses by an older head unit will have to be estimated as well. There may be little or a lot depending on it's condition.

If I wanted a pretty accurate estimate. I would take an air tool with a known rate of consumption. Run it off the comoressor tank while watching the pressure gauge. If the unit could maintain pressure for say a minute or more then use the next tool that has a higher rate of consumption. Or lesser if the first one tried was too much for it.

Actual delivery would be found well enough that way at a given pressure. Most air tools have their consmption posted somewhere at a given pressure.