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<p>Robert,</p>
<p>Can a make a second attempt to make more sense? But this is, I am sure, boring to you.</p>
<p>The group that I do deep dives with use a thirds approach: whether you dive caves or open water and nothing goes wrong, diver needs a third of the back gas remaining at the end of the dive. So, given SAC rate(s) it is simple (but sometimes tedious) to calculate the cylinder pressure at which ascent starts. But ascent pressure is not merely a third, diver and buddy must be able to get to 1st cylinder change depth, so this amount is added to the cylinder pressure value of a third. This often gives ascent start pressures between 80 and 100 bar. It would be useful if this pressure at start of ascent were calculated by Subsurface. The reason why this number is useful is of course when one underestimates SAC for that particular dive and the bottom time should be shortened because of remaining back gas requirement. This number goes on my slate every dive, therefore the usefulness. Do I make sense at all this time round? Good communication is difficult. Kind regards, willem</p>
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