Hi introducing myself.

[slangbela]

The gauge consists of 1/4"sheet of Perspex mounted exactly perpendicular to the lathe bed, with a fine scribed line on each side at center height. Looking down on the mirror, mounted ar 45 degrees, move your head until both lines coincide. If the lathe tool is brought up close to the line, You can see through the Perspex the center height relative to the tool edge..