 one third of the capacity of the cylinder is considered a simple rule of thumb reserve sufficient for most circumstances, but there are times when it is not enough and times when it will be more than enough.The principle of the rule of thirds is that one third of the gas supply is available for the penetration, one third for the return and one third is a reserve which allows for gas sharing on the return.Therefore, the dive is turned when either cylinder drops to two thirds of total capacity.