mempilot:True. But I figured they wanted credit for the shallow portion of the dive. To plan a dive to 60' because you plan on hitting 80 for half the time and 40 for the half the time doesn't really make much sense. Planning a dive to 80' and only spending half the time there and the other half at 40 isn't efficient planning. Planning a multilevel dive at 80 and 40 would make sense and be efficient. It would also allow one to know their N2 loading more precisely for repetitive dives. Balancing the depths for use in the charts is not supported.
You are not describing proper depth averaging. Plus, I generally ignore any time spent above 50' on a recreational dive anyway. (Disclaimer for the idiots: there are certain guidelines that have to be followed to do that.)