That brass gage is useless for choke. I had a 16 ga. that the barrels were cut, someone took one of those choked gages and it stated full.
First, if you don't know the bore than you won't know the correct choke.

I have a Skeets gage that can read 16" into the barrel, read what it states then zero it and read the constriction. If you are not math-challenged, don't zero it, but I do.

David