I've done a few of these over the last two years. The method uses vector geometry and the cosine rule. See
I think Wiki also gives the co-ordinates of the points in 3D so you can pick two adjacent faces, get the critical lines and apply the method.
I've got a post to make for MIF, so It'll be a while before I can put all the figures to this, so have a go yourself.
ps. I have a vague memory that I have done it in a thread, but I couldn't find it.