My rain gage got it’s first test the other day. We got a nice 1 7/10" of rain in there. The water line is just barely visible under the bottom holder bracket.
And yes BillyBob in Kansas, your rain gage is metric because it measures in tenths of an inch. So take that! You commie pinkos in California!
As a side note: We got a light frost last night. It got down to 31F/-1C at some point last night. Not a hard enough frost to kill the bell pepper plants outright, but it did singe and tip the leaves a bit this morning.
The “standard” is whatever units are called for. If the customer wants it in cubits, they get it in cubits. Again G20/G21.