So what do I do to "plan" the dive? Nothing really. Basically we all gear up and off we go watching our computers. I quit logging dives when I reached 500 so I do have a bit of experience and pretty much understand how much air I typically use on a dive. I don't think I have ever seen anyone, myself included, actually take a computer, go to the planning stage before the dive, and run through a program that tells you how much time you will have at 100 ft. Do I really need to know before I jump in how much time I have at 100ft? If I want to know that I just look at my computer when I reach 100ft.
The second dive will be much like the first. You get to the site, get the briefing, and will be told something like, "The max depth on this dive will be 50 ft. and the dive time will be 60 minutes. Watch your computers and be back on board with 500 psi." Again, I have never seen anyone on board take out their computers to see how much time they should get at 50 ft. because in a couple of minutes when you are at 50 ft. you can just look at your computer and see.
Just because you don't see it, doesn't mean it's not happening. It's not a terribly complicated or time consuming thing to look at my wrist, push a couple buttons, and see what the NDL for our next dive is. I certainly do it, as part of my gearing up process.