Back when I took the test, I do not believe that prop slip was a part of it. The formula given by JEFryar is the easiest way to get it. You can basically think of it as a measure of how far you should have gone(determined by the pitch and the rpm) and how far you went(determined by boat speed) expressed as a percent.
For those of you debating between studying on your own and taking a course, unless you are a really good book learner on a budget, seriously consider taking a course. You will end up spending less time in the end and it is a really good way to meet other people interested in doing what you want to do. Check into the specific school that you are thinking of beforehand because there are stories of really bad teaching but also some of really good teaching. I was not going to take the course but got talked into it by a friend and feel now that it was definitely the right thing to do.
Out of curiosity, those of you looking at license, what are you planning to do with the license? Are you actually going to put it to use or just have it and consider it a good educational experience?