Questions Concerning The 2c Harding Stamp

So I've recently purchased this perforation gauge from Amazon. Seems to be working just fine for me. Ok..

Questions: 1. First, am I actually taking the correct measurements/counts with placement of the stamps?

2. Does the size difference shown make this a possible 613? If not, which else is required?

3. Lastly, what's the best way to accurately identify stamps using this Lighthouse gauge?

**I used the method mentioned on Linns where you cut the flat plate stamp vertically in order to use as a tool to determine whether the two were of different size**

Thanks in advance for any feedback and information!

One thing you need to do is set the left edge of the zero line at the design edge. You have gained .5 mm by not doing so.

Ok cool thanks Rod. I was having trouble finding my placement