Thread Check Inc offers both horizontal and vertical tapered blocks specially designed to calibrate tapered plug gages. Instructions are referenced below.
INSTRUCTIONS FOR USE OF HORIZONTAL TAPER MEASURING BLOCK
In order to check the Major Diameter, the gage block setup is the small end major diameter divided by 1.000489. Then add the constant, marked on the block.
In order to check the Pitch Diameter, the gage block setup is the small end pitch diameter plus the best wire constant and that total divided by 1.000489. Then add the constant, marked on the block.
Place the block on the measuring device, with the back of the block against the floating anvil. Place the plug, to be checked, small end down. Move the plug into measuring position. You must now square the floating anvil to the measuring block. Turn the block slowly, first left and then right, until you achieve the smallest reading possible. When you reach the smallest reading, the block will be square. You may now take a measurement.
A hold down clamp has been provided but its use is not recommended unless necessary. The plug gages will sit on the small end without issue.