Several years ago I finally got around to creating a short text file on my hard drive where I can jot down my inner rim widths. Then, if I am curious I can look it up.
Or, if buying Velocity rims, use the chart on the bottom of this page.
https://www.velocityusa.com/tech/rims/
I think the Velocity chart looks pretty good. But you have to be careful, some rim manufacturers have outlandish claims for how their rim can use almost any tire ever made.