Your math isn't quite right, in that the ascent and definitely the safety stop are not done at the max depth.
In addition, for emergency purposes you should assume something higher than your normal SAC.
-----------------
Here's a couple of calculations:
Normal air consumption, 1 diver, 0.5cfm SAC, 30fpm ascent from 60', 3 minute safety stop.
Ascent is 2 minutes (it doesn't matter that part is before, and part is after the safety stop0; average depth 30'. avg ata approx 2.
2ata * 2 minutes * 0.5cfm = 2 cu ft.
Safety stop depth of 15' is approx 1/2 atm, or 1.5ata.
3 min * 1.5 ata * 0.5 = 2.25 cu ft.
So normal ascent and stop at 0.5cfm is about 4.25 cu ft.
------------------------------------------
Now lets do you scenario. 2 people, 0.5cfm each, 2.5 minute ascent, 2 minute safety stop.
1cfm total. average depth for 2.5 minutes of 30', or about 2ata.
2*2.5*1=5cfm for ascent.
2 minutes at 1.5ata safety stop is 2 min * 1.5ata * 1cfm= 3cfm.
8cfm total. For an AL80 it is just under 40psi/cu ft, or about 320psi used. Add 500psi reserve and you get 820psi rock bottom using your asssumed 0.5cfm and no time fixing a problem at depth.
------------------
I don't agree with your assumptions however. I would assume a much higher consumption rate of 1 cfm per diver in an emergency, and assume that it takes a minute to sort out things on the bottom.
1 min x 2 divers x 1cfm/diver * about 3ata = 6 cu ft used before starting ascent.
Ascent from 60' at 30fpm is 2 minutes. 2 min x 2cfm x 2ata = 8 cu ft for ascent.
3 minute safety stop at 1.5ata is 3 * 1.5 * 2 = 9 cu ft.
6 + 8 + 9 = 23 cu ft. 23 cu ft x 40psi/cu ft = 920 psi + 500 reserve = 1420 rock bottom.
In reality, I wouldn't add in the safety stop ---- in real life, if we are so freaked out that we are both breathing at 1 cfm, then I'm heading on up to the surface rather than doing an optional safety stop. OTOH, if we aren't breathing at 1cfm each, then we'll have enough gas for the safety stop.
Dropping the 9cf requirement for safety stop just leaves 6 cu ft to get sorted out on the bottom, and 8 cu ft for the ascent. 14*40 = 560psi. Add in another 500psi for margin if you want, to get 1060psi.