RJ-
I think you might be forgetting to add in the time it takes to get to your 70' stop into the backgas calculation.
Tom
65cf was my rock bottom for 150ft from memory. Here it is laid out step by step:
1min for the failure on the bottom
1min up to 120
1min up to 110 (conservative rounding)
1 at 110 (a bit deep but officially part of the 80% of ATAs or 75% of depth rule)
1 at 100
1 at 90
1 at 80
1 for the switch at 70
Total = 8min ascent
Average depth = 105ft = 4 ATA
4ata * 8min * 1cf/min = 32cf
32cf * 2 divers = 64cf rock bottom
Your normal ascent should use less gas
4ata * 8min * 0.5cf/min (typical deco SAC) = 16cf
64cf - 16cf = 48cf remaining which is roughly the amount you need to deco out on backgas if your 50% were completely empty and you had no buddy. You need 54cf to deco out on backgas at 0.75cf/min but only 38cf if your deco SAC is 0.5. Somewhere in there is the true value which you won't know until you're actually in this (remote) situation.
I use 66cf/1100psi as rock bottom in double 100s although I rarely get that low on a T1 dive nowadays. Really all the math is just an educated guess at what kinds of reserves you need. As Joe T would say "its scuba math", don't use a calculator.
For bigger "T2" dives with experienced buddies we don't use a stressed SAC of 1cf/min because the reserves become obscene and we know we don't use those kinds of volumes stressed. If we did get that stressed, we probably shouldn't be doing 200ft dives.
By all means carry larger reserves if you feel you need them, and calculate your turn/ascent pressures accordingly.