The Plus12 is a measuring device for both shoes and feet.
Stand barefoot or in close-fitting socks on the Plus12, so that the red arrow is a bit longer than the big toe. Adjust to feet and step off to see the measure. You have the correct inner length for the shoe, including 12 mm extra for proper fitting.
Insert the Plus12 into the shoe until the red arrow touches the point of the shoe. You can now take the measure.