Cutting the posts proved a little problematic at first. 

The first thing I do is slip my vinyl fence post cover over the post and use my laser equipment to set the post height.

Next, I add a clamp snug up against the bottom of the cover.  This puts the clamp exactly 1.5" below the point to be cut.

The saw blade is exactly 1.5" from the short edge of the guide plate.

All in all it worked very well.