Those paired white and blue boxes in Table 3--a simple memory aid I offer my students for them to remember which box is which is that we think of air as being colorless, so the box with no color (the white one) represents the residual nitrogen time (RNT) after a dive, and we think of water as being blue, so the blue box represents the actual maximum allowable dive/bottom time (ABT) at a particular depth for a particular pressure group. If you add those two numbers up, you get the exact same number as in the black boxes in Table 1. RNT (white box) + maximum ABT (blue box) = NDL (black box).
The only time we use the RNT number in the white box is when we are figuring a pressure group after a repetitive dive (second, third, etc., of a series of dives). In that case, we add the RNT number in the white box to our Actual Bottom Time (ABT) for the dive we have completed to produce our Total Bottom Time (TBT). Our new pressure group is derived from the TBT by following the rows and columns in Table 1. In other words, when we start a dive with RNT remaining, we calculate our TBT as if we had already spent that amount of time in the water.
