Boyles law.

The rated 30cfm is for FAD (Free Air Delivery), which is basically what it can provide while spinning freely while generating no relative pressure.
Now take Boyles law that states that provided the temperature remains constant, the volume and pressure are directly proportional, i.e. as you increase pressure, you proportionally reduce volume.

As atmospheric pressure is around 15psi (14.7psi to be exact, but it varies depending on height above sea level/weather), the compressor can pump out 30cfm at 15psi (this is called absolute pressure - a pure vacuum is 0psi absolute).
So for example if you then require the compressor to double the pressure to 30psi (or 15psi relative to atmoshperic pressure), the volume of air produced drops by half, provided the temperature remains the same.

As we need 75 psi (I've rounded this up for easy figures!), we need the compressor to produce 90psi absolute pressure, which is 6 times the FAD pressure, so FAD volume then drops by a factor of 6, which gives us 5 CFM at 75psi.
And now I've just realised my calculation earlier was wrong!


Those figures don't allow for temperature change, which will affect the actual figures, along with the compressor effiency, but they still give a rough idea of what the compressor can deliver.