I just used this tool to measure mine, and while it did the job, it's a very clumsy tool in my opinion. The way in which it assembles caused problems / interference right at my needed wheel/tires widths (10" and 11.5"). We had to disassemble and reassemble it a few times to get any adjustability in the ranges we needed...
If there's a better option, you may want to look for it. While this one worked, I wouldn't have wanted to pay for it...
In that last picture, you can see where the black wingnut was 'bottomed' out against the bracket on the very left. I couldn't just revert that l-shaped bracket because then it hit the other screw. I could put it on the other side of the big flange, but then it doesn't measure correctly. The first time I did that, I was off by 3/4". Thankfully we put it all back together again and realized I was miscalculating because of the flange being on the other side of the bracket...
Anyway, like I said, it was a little clumsy to use. I'm sure once you get the hang of it, it would be fine...