That is the theory as far as how PR gets split up, although there is also the issue of non-related links being discounted altogether, so it may not be just a simple calculation.
There are a couple of sites out their that are cited as credible information for helping to figure out how PR is calculated.
Check out:
http://www.kenkai.com/google-pagerank-table.htm
http://www.compar.com/infopool/articles/PR-calculation.html