The pressure to volume relationship is essential linear. Definitely close enough for SAC purposes, although gas blenders may use some correction factors.
Frank O gave you the basic way to get cubic feet of gas used.
For easier calculation of expected gas usage at different depths, air consumption is normalized back to what it would be on the surface, hence the term SURFACE air consumption.
At 33' the absolute pressure is 2ata and you deplete the tank at twice the rate as on the surface. At 99', you use up the tank 4 times faster. Use the correction factor of (Depth in feet + 33)/33 or (depth in meters + 10)/10 to compensate for depth.
Putting it all together, if we assume that you went from 3000-2300 psi on an AL80(which is really 77.4cu ft at 3000) at 40' ----
(3000-2300)/3000 * 77.4cu ft * 33'/(40'+33') / 20min = .41cfm
(startpsi - endpsi)/ratedpsi * ratedvolume * 33'/(depth+33')/divetime = SAC
For the depth, you have to use the average depth for the period you measure gas consumption. So either do it when at a constant depth, use a computer that give you average depth, or note your depth throughout the dive and average it before calculating SAC.