Kevin,
We know how much gas we have assuming that the tank was filled all the way.
Now we need to know is the Surface Air Consumption (SAC) rate. That way we'd know how long the tank would last. Then look at the dive plan at a certain depth (say, 100-ft) and see how long one can remain down there until reaching No Decompression Limit. Then calculate to see how long it would take one to surface (what ascension rate does the diver want to use; 20FPM, 30FPM, 60FPM), take into account of a minimum of 3-minutes safety stop and also take into account of leaving around 500-psi in the tank for safety reserve.
Check out this website
Determining SAC Rates
The formula for determining the gas requirement is as follow:
Cubic feet required = (minutes X SAC rate) X [(depth in feet + 33) ÷ 33]
Let's use my own example. As you know, I use the HP100 at 3150-HP. So assuming that the tank is fully filled and I have 100-cuft of air available to me. With 500-psi for reserve, that equals to 16-cuft (rounded up from 15-something). So that means I actually would have 84-cuft to work with. This is a simple ratio equation because everything is pretty much linear: 500psi/3150psi = X-cuft/100cuft. calc it out and you get X = 16cuft.
100cuft - 16cuft air reserve = 84cuft to play with.
I'm going to a depth of 100-ft and the PADI air table gives me 20-minutes until I hit NDL. My SAC rate from my dive history is around .75cuft/minute - not spectacular, but hey, I'm an air hog and a fat dude. Using the formula above shows:
CuFT required at 100ft = (20min x .80cuft/min) X [(100ft + 33ft) / 33ft] = 64.50cuft
CuFT required at 15ft safety stop = (3min x .80cuft/min) x [(15ft + 33ft) / 33ft] = 3.50cuft
Thus far my gas requirements are already at 68cuft out of 84-cuft available to me (if I were to want to have the 500-psi reserve at the surface). That means I have 16-cuft available for descend and ascend. 16cuft/.8cuft/min = about 20-minutes for descend and ascend.
Or just buy an air integrated computer and go for it. Haha.