When you use your SAC to calculate how much gas you expect to use on a dive how do put a fluctuating value into your calculator? if as you assert SAC is not a diver specific constant.
It’s not like SAC calculation is difficult.
You need a note of your start and end pressures. Pick some square profile dives and work out the averages.
The easiest way is a spreadsheet of depth in metres for each minute of the dive then divide by 10 and add 1. E.g. 30m / 10 = 3 + 1 = 4ATA.
Work out the volume of gas consumed = (start pressure minus end pressure) x cylinder wet volume.
Then divide the volume of gas by the total pressure minutes.
Then you’ve a baseline.
With that baseline you can work out you higher workload (x1.5 to x2) and extreme workload (x3)
It’s those higher consumption figures you use for working out minimum gas and having sufficient for contingency, maybe even gas sharing