Turn pressure – simply put.

The simplistic method is divide the starting pressure by 3 rounding to the conservative side.  For example, the case

where you had 2900 psi you would start with the first pressure evenly divisible by 3. In this case 2700 / 3 = 900 psi is

your third. You then take starting pressure and subtract the third for your turn pressure or 2900 - 900 = 2000 psi.  You

should then end the dive with 2000-900 = 1100 psi.  The thirds rule is used when you have to get back to your starting

point. In open water you can state the rule as 'start your ascent with 1/3 of your gas  supply remaining.   The more

sofisticated version is refered to as gas management with  dissimilar tanks. The concept is that you have to get both you

and your buddy back with only one set of tanks. So it depends on both of your SAC  rates and the size of tanks. So if

you have a hoover for a buddy and you are  using small tanks your turn pressure will be higher than a third.