A tire tread depth gauge is handy for checking freeplay of a chain. Goes up to one inch and has metrics.
Camel Tru-Flate Tread depth gauge 40-395
by Camel
Link: https://amzn.com/B000BYM4VS
Also good for measuring other things.
If you don't know your own strength when flexing a chain by hand then here is a good reference.
Get a big paperclip and make a hook on one end that will hook a roller on your chain. Tie a string on the other end of the paperclip and dangle it down and make a loop and hang a plastic bottle with one pint of water in it. Hook the paperclip in the mid section of the chain.
Now you will always have a reference of 1.04 pounds of pull.