I agree with ScottG to an extent. These numbers aren't 'drop dead' numbers where you use up every last pound, but cannot go over.
In reality and for practical application, there is no functional difference if one is a few hundred pounds over or under the stated limits. In the end, it is up to each person how they treat the limits, but a specific calculation of precisely where you are in relation to the limit is not really necessary.
Note -- I'm not saying don't weight your setup and get an idea of what your weights are, etc. That is good information to have. But what I am saying is that if one is worried about the last couple hundred pounds on a 15,000# gvwr, well, it isn't going to make any difference in my opinion.
