Thanks for your interest, Mr. EOU. 
My goniometer was inspired by a book written by prof. Verhoeven and also by handbooks to Catra goniometers which Mr. Wootz pointed out to me.
I do not have a sophisticated calibration procedure. In my goniometer the blade is mounted in a small vise which allows sideways movement of the blade. I assume that (at least) the blade is symmetrical and try to adjust the centre of the shadow cast by the blade on the division 0° of the scale. Adjustment accuracy is circa 1°. This approach allows me to measure asymmetrical bevel angles also.
Jan
My goniometer was inspired by a book written by prof. Verhoeven and also by handbooks to Catra goniometers which Mr. Wootz pointed out to me.
I do not have a sophisticated calibration procedure. In my goniometer the blade is mounted in a small vise which allows sideways movement of the blade. I assume that (at least) the blade is symmetrical and try to adjust the centre of the shadow cast by the blade on the division 0° of the scale. Adjustment accuracy is circa 1°. This approach allows me to measure asymmetrical bevel angles also.
Jan