Calculate and emit "sameness" percentage

When comparing two directories with a large amount of files, I'm finding myself wanting to emit a "how much of these two directories are the same" as a calculation. This would help in making decisions on whether further analysis is required - if two directories are 99.99% same, maybe further analysis isn't needed, or maybe specifically there is to find out what the 0.01% change diff is - either way, it's something that could be used to make decisions.

I check the output from --json flag in hopes that there was something I could use there to caluclate a percentage, but nothing stood out to me.

Is this kind of feature desirable? If so, could you point me in the direction of where that might be added?