For obtaining accurate angles, I've found the Wixey WR300 gives me angle measurements that are consistently within 0.1° of the frame manufacturers' published angles and the results are repeatable.

Start out by positioning the front and rear dropout centers parallel to a flat surface (I use aluminum box tubing). If the wheels have exactly the same diameter (wheels are true, same rims, same tires at the same air pressure), the dropouts will be parallel to the flat surface. By "zeroing" the WR300 while it's on the flat surface, the HTA and STA relative to the dropout centers are easily determined.