IKEA Lack hanging helper

I was tasked to hang 16 of these IKEA Lack shelving units. These are the small shelve units. Instead of measuring 22.5cm for each install, I designed and printed a guild to show me the hole spacing and a tab to show where location of top surface of shelf would be.

Once you decide where the top surface is to be, Place tool against the wall with the tab at the correct height, use a small bubble level to level it, mark the holes, drill and repeat.