Product review (2 stars): Not really calibratable
Purchased (3) of these.. One came with a broken glass. Poor packaging by Amazon (nothing in the box but the items). They are replacing it.

But the real question is calibration? If you can't calibrate them they are useless. The two I got are off by 10% from each other.

There IS a screw slot on the back but turning it turns the entire instrument in the case, including the scale. Removing the back allows one to carefully hold the scale and move the mechanism to calibrate it. Also, Amazon gives you no place to click and ask a question about the product. Grump Grump.  I'll update this once I get the replacement and see how they calibrate etc. That may add one star if it all works. The description should be changed to "un-calibratable for the average user". They are really nice looking.