Your gas consumption is SAC times pressure times dive time. Are you spending your entire dive time at "average" pressure on a square profile? How about a triangular one? A see-saw?
I just don't get the notion of "average depth" -- is it max depth divided by 2? -- so I find its lack not odd at all.
No... it is
(each depth you were at during the dive) x (time spent at each depth) / total time of dive
Essentially that is how we define the average value of a function. If the function is continuous, we chop the time into tiny bits and the average becomes
average depth = integral of (depth as a function of time dt) / total time
If you were at one depth for half the time and another depth for half the time, then yes... the average depth is just the average of the two depths. But that principle can be generalized (as described above) to any number of depths for any amount of time.
Even better, because pressure is a linear function of depth, and air consumption rate is directly proportional to pressure, after calculating the average depth for any dive, as described here, you can easily calculate your SAC by using:
SAC = (start pressure - end pressure) x [(average depth in feet / 32.8 feet) + 1 ] / time of dive
where the "average depth in feet / 32.8 feet" will give you the pressure, in atm, due to the water.
If your start and end pressures are in psi, and time of your dive is in minutes, you'll get a SAC in psi per minute. You can easily convert this into cubic feet/min by multiplying by
(capacity of your cylinder in cu ft / pressure of your cyl when full in psi)