Depends on the computer . . . some start to give you "credit" for offgassing as soon as you hit the offgassing ceiling, and others will continue to accrue deco until you hit the stop depth.
To the OP: If you want to understand more of what is known (and theorized) about decompression, Mark Powell's book is excellent. I would also recommend the GUE DVD, "The Mysterious Malady," which consists of interviews with the folks who are actually involved in research in the area. It's both informative and cautionary. You can also purchase the UTD online course on Ratio Deco, which, although some of the things presented as fact there are actually controversial, is still a reasonable overview of different approaches to planning decompression. I would also highly recommend NW Grateful Diver's article on
Gas Management, which introduces the very basic ideas of gas planning, which is a critical component to any dive where you intend to incur a decompression obligation. Once you have lost the surface as an option to solve problems, you HAVE to have enough gas to get you through the time you are forced to stay underwater. Knowing how much that is, and planning for contingencies, is simply critical.
If you are actually interested in the mechanics of planning and executing staged decompression dives, you need a class that is pragmatic and experience-based. It's not a bad idea to get some insight into the level of skill required to cope with problems underwater, as well. None of us here knows your diving competence, but among the things you should be able to do if you are going into deco is all the basic OW skills (except the relatively silly ones, like removing and replacing your kit) done while hovering, without losing position (don't want to lose the upline!) or changing depth (can't blow a deco stop because your mask floods). The Fundamentals class which has been mentioned here is a very good way of assessing your abilities in this arena, without either the expense or the risk involved in taking an actual technical diving class. Fundies can be done in a single tank.
