Review on Precision 10x10x29mm Picture Measurement Tools with Hanging Capability by David Gagnon

As others have reported, they are not very accurate. The photo is mounted on a calibrated, precisely aligned plate. They either need to have some mechanism to customize what you're going to use them for, or hope you get a good one in-game and have a way of finding out. They are cheap and the packaging is convenient, but can be disappointing if you cannot solve the quality problem.