A better measure is what % of the total money supply you have.

I.e. you started out with 2e-20 % of the total money, and after 5 years you now have 1e-20 % of the total money, then whatever happened to CPI, you've been diluted and you would probably have been better off investing in something else other than cash.

That makes sense in theory, but in reality what "total money supply" is is a complete can of worms and basically impossible to measure