perspective problems

I am using photoshop to figure out how many different vanishing points were used in a composite painting, based on one section. After I select the section and choose 'free transform' and then 'perspective', I noticed that I can also shift the point of referece (the icon in the center of the box) but I don't quite know how I can best utilize this feature. Should I keep it centrally located or might it be helpful to shift it. It's important to keep some kind of consistency, of course, but I'm not sure what kind of difference it would make if the p.o.r was shifted.

Hmm... I know this isn't a helpful reply, but I've never tried shifting the centre point when perspective transforming, I'll do some experiments, post any interesting results you have.