An AL40 does last about 30 minutes of deco as a 70 bottle, but there is more to it.Minimum stop times are applied to the various depths from 0 - 30, 31 -60, 61 - 90, 91 - 120, and 121 - 150 minutes of deco at the O2 Segment. Using Cascade Deco, you determine how much deco you need on O2 at 20' and then figure out the rest of the deco based on that. From there, depending on how long the deco segment is, determines the minimum time for each stop and even how you curve the deco. Your Cascade Deco document, while mostly accurate, leaves out a lot of information. You can also use it for dives shallower than 150'.
For example. On at 400' dive for 150 minutes, you wouldn't do 2 minute stops at each deco stop and then finish off the deco segment with the remaining time. Nor would your deep stops be one minute long. Your total run time is going to be about 10 hours after you take into account for ascent to deep stops, the deep stops, the deco at the four different deco segments, and a six minute ascent from 20.