Ok Phil what you call the toolpost is a banjo, It is already 90 degrees to the lathe bed you cannot change that angle.
The angle you can change is the top swivel angle, assume you are looking down on the lathe from above you have your stem standing up at 90 degrees to the lathe bed the top is facing you with its own little bed flat.
Imagine that bed square, turn it so one of the faces is toward the sandpaper, hold a thick piece of paper against the sandpaper, now push the bed up to the sandpaper and tighten the clamp on the banjo pul the thick paper out and you have faced up square to the sandpaper. Check if you like it will be 90 degrees.
Here's a couple of usefull links to give you the idea of how you are going to sand against Terry's jig.
http://www.google.co.uk/url?sa=t&rct=j&q=sanding%20disc%20squaring&source=web&cd=1&ved=0CC0QFjAA&url=http%3A%2F%2Fwww.penmakersguild.com%2Farticles%2Fblanksquarejig.pdf&ei=y08FUsvCJoaChQeoooHADg&usg=AFQjCNFhYcNSwjEbVdKAI4QpVSYA_qekVw&bvm=bv.50500085,d.ZWU&cad=rja
Using lathe to sand square blank ends - International Association of Penturners
Wood lathe disk sander - YouTube
