I have read the manual a couple times. I am going to sign up for the DIV Nav class when I get a chance. I did not see the topic addressed. The manual is very detailed.
The fact that the manual is so detailed (and all of them are) is a key part of the problem. A key concept in information theory is that
too much information is the same as too little information. You don't need to know the overwhelming majority of the information in that manual, but they have to put it all in there to cover their buns. The information you need is lost amid all that other stuff, which is classified in information theory as "noise."
In the computer version of the PADI OW course, students are taught that
all computers have several key functions--really only a handful that you truly need to know. Once you get your computer, you should go to the manual and learn how that model works with those specific functions. You can safely disregard pretty much all the rest. Dive planning is one of those functions, and a new computer owner should pick up the manual, turn to the table of contents, and look for the section that explains how that works.
As long as I am on that topic, another one of those key functions is
emergency decompression. Your computer will tell you how to get to the surface safely if you violate NDLs, but you have to understand what it is telling you to do. I was once working with some students, and several other divers from my group were diving on their own at the same time. They ended their dives when we did, and as soon as one of them reached the surface, he started talking about the weird thing his computer did. When he was at the bottom, he noticed that it was giving the some signal he did not recognize, and it was counting minutes up instead of down. I did not know his computer, but I immediately recognized that he must have passed NDLs, and the computer was telling him how long his decompression stop needed to be. I checked his computer, and it was in error mode. He did not remember what numbers it was showing when he surfaced. I took him back down to 20 feet and we did a very, very long omitted decompression stop. When we went back to the surface, I put him on oxygen just to be safe.