If you can believe it, tools of a similar nature to this one retail for nearly $30 in industry-specific catalogs. Sometimes you can find folk attempting to charge as much as $50 for this simple piece of plastic. Deviant Ollam could not personally believe that this was actually happening and he worked to design a smaller and more functional version of these useful tools.
Dev's Door Gap Tool is more compact and also supports being carried on a keychain or lanyard. Whether you're performing real-world NFPA 80 fire door inspections or you're merely adopting the auspices of that cover story to explain your presence in a sensitive area during a physical penetration test... this tool can help you measure door gaps and latch throw distances.
The Red Team Tools Door Gap Tool has measurement surfaces in the following sizes:
Some photos here show white pigment augmentation which is not included by default but which some users opt to DIY using a rub-on paint stick in order to enhance legibility of the characters which are debossed into the gauge.