
Forgive my ignorance but...
I believe the PCD is the diameter of a circle which goes through the centre of the 5 bolts. This can be done by measuring the radius and multiplying that figure by two. i.e. centre of the wheel to centre of a bolt x 2.
I have been told another way, which applies to 5 stud wheels only, is to measure the distance between the centre of two adjacent bolts and multiply that by 1.7012.
Just my $0.02