[kelnos]

Ugh, so I was just making some new window screen frames to replace some that were missing, and what a pain that was to measure things out. I had fun dimensions like 21-5/16" all over the place. Now, they certainly didn't need to be accurate to a sixteenth-inch, but being off by even a quarter inch could easily make them not fit.

Too large and you just can't get the screen into the frame, and too small and it can just fall out, or at the very least leave gaps large enough that bugs can get through.

I agree with you that there are many things where we don't have a specific physical need for certain dimensions, so we just pick round numbers. But I think the cases where we do have to conform to some messy existing physical dimensions come up more frequently than you think.