P802 Digital Depth Gauge Holder

Used to mount one of these http://www.banggood.com/Digital-Tyre-Depth-Gauge-Tyre-Tread-Depth-Gauge-Measuring-Tool-p-953762.html on the rails to check the level of each corner.

Zero it by pushing it against the bed before mounting. It's best to sand down the end so it's nice and rounded to stop it scoring the tape you put on the bed.

I'm sure you can work out the rest....

Tip: only use a feather like touch to push the ruler down or it will push the bed down and you'll get a false reading