What are the issues with measuring chain wear?
The biggest is the diameter difference of SRAM's flat top chains found on most of their modern drivetrains. The rollers on these chains are larger, enough to account for a .2% difference. When we're replacing chains between .5-.7% of wear that .2% can easily be the difference between replacing just a chain and an entire drivetrain.
Most chain wear tools currently on the market are laser cut metal. The accuracy of this process isn't great at +/-.010" (.254mm). If you add the error of the tool to the roller variable it's possible to double the error of the tool.
The Abbey Chain Wear Plate is a tool built to handle the variance of roller diameter when measuring linear chain wear but also measure lateral wear.
To measure lateral wear, the chain will need to be off the bike. Then simply pinch the chain against the boss and hold it vertical and see how far it sags.