The height ratio on the diagram is based on an arbitrary girdle thickness of 2% of width. Ignore it, as Frankwood says, and cut to the thickness specified. For measurement, there are magnifiers with measuring reticles available. I have an excellent Bausch & Lomb Lenscope, unfortunately no longer manufactured, but they may be available used. They’re pricey. I got mine many years ago, at employee price.
What many people do is find a wire or drill bit of the proper thickness for visual comparison. That’s plenty accurate enough.