Hi Stuart,
Right now I do it manually. Diving an AL 80, if I am shallower than 90', I leave it at 500 reserve. For 90 to 120, I go to 700, and 120 or more, 800--I set it for less if I am on an HP 100 or 120. Maybe more if it is going to be a stressful dive (night dive in a strong current for example) Like boulderjohn said, in a rec diving gas sharing emergency I am not doing a safety stop, as I expect my OOG buddy's first priority is to get to the surface. Similarly, I am heading up at 60'/min until I hit 30 feet when I will slow it down.
What you seem to be posing is a "rock bottom" algorithm, where the computer calculates ATR based on the gas needed for ascent based on two divers using the tank--i.e. double your current real time consumption at the current depth (the only one available to the computer), as predicted through the ascent (like current ATR algorithms). This would be "rock bottom" refined for the real-time conditions of the dive and would replace a pre-set "reserve". That might actually be a good option to offer, especially if you could program the parameters of the ascent (no stops, faster rate as I would do, or true "rock bottom" with standard ascent and the stop).
No, I am not posing a rock bottom algorithm. I am just suggesting that what you described that you do manually could be done automatically. Instead of having 1 hard number for your reserve, it could automatically vary the amount of the reserve based on your current depth and dynamically update your ATR based on that reserve.
E.g., in your case, your computer could use 500psi as the reserve as you descend and swim around at depths down to 90'. If you descend further, it would start increasing the reserve number that it using. So, your ATR would drop as you descend. It would drop as you descend anyway, but what I'm talking about is that it would drop faster than would be accounted for solely by the increase in ambient pressure. It would drop faster than that because the ambient pressure is increasing AND the reserve number it's using is also increasing. Conversely, if you ascend, your ATR might increase (or increase more rapidly) as the reserve number used in the ATR calculation drops.
The more I think about it, the more I like the idea. Currently, my computer has one setting for Reserve, which has a default of 500psi. The "enhancement" would be to add one more setting. The current setting would be the Reserve for any depth of 60' or less. The new setting would be the Reserve for a depth of 130'. The default would be, say, 1000psi.
So, right out of the box, if I dive to 130' and hang out, the ATR calculation will be sending me up to the arrive at the surface with 1000psi. If I dive to somewhere in between, say, 95', it would interpolate the 2 numbers and calculate ATR based on a reserve of 750psi.
Hitting the surface with 1000psi may sound excessive. But, one, that reserve would shrink as you ascend, so you could stay down longer, if you are staying at a shallower depth, and you'd arrive at the surface with that lower reserve. If you ascend at hang out at 60' or less, and then ride your ATR, you'd still arrive at the surface with 500psi.
And, two, you could change the default. So, you could make the reserve for 130' be 800psi or whatever, if you want.
And if you're diving a bigger tank, you could change both numbers to lower values, if you want.
It's really just about eliminating that manual step that you are doing now where you adjust the reserve based on the planned max depth. And for computers that let you enter the cylinder size you're using, it could even automatically compensate for that. If you're set for 500psi on an AL80 and you change the computer to tell it you're using a 100, it would reduce the reserve setting so that you're hitting the surface with the same amount of air left, though it's a lower psi number.
To the Negative Nancys: I have not and am not saying anyone NEEDS any of this. I'm not sure why I'm repeating myself, though. I've said this enough times already that I can only include some people have a fundamental lack of reading comprehension. Either that or PermaGrump. Or both.