FYI:
If you ship UPS they do not measure girth as you suggest. They require you to measure width, round and double, measure length, round and double, and then add the two together. This quadruples your rounding error. If you don't do it this way and they do and come up with a girth that is too large you will incur an oversize package fee. I know because the same thing happened to me and I had to contest the charge. Don't count on any shipper to do you any favors. Here is their measurement policy page.