You might try Brownell's to find a tool that used to be nicknamed a 'Sinclair Nut' that was made by Sinclair's before Brownell's bought them out. It looks like a big hex nut that has different bullet sized holes on each of the 6 faces. You use it with a caliper to measure the length of seated bullets to check chamber touching or freebore. They also make one to use for measuring the case length, just like you are trying to do now.
https://www.brownells.com/reloading/...prod83792.aspx