I downloaded Microtype Typographic ruler form

http://www.microtype.com/typometer.html

and I am trying to understand how to use it. The PDF has a note that reads.

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This font is Helvetica, where the measured cap height is 72% , and the x height is 52%, of the specified font size.Thus when you select Helvetica 20 point, the cap height is 14.5 points, and the x height is 10.5 points.
This proportion can generally be regarded as “standard”, and the sizes shown can be used with most fonts.
Following are cap height percentages for some common fonts (these might vary according to font manufacturer):
American Typewriter 66%
Antique Olive 75%
Avant Garde 75%
Benguiat 66%
Bodoni 70%
Bookman 69%
Century Schoolbook 72%
Clearface 69%
Franklin Gothic 70%
Futura 75%
Garamond 62%
Gill Sans 67%
Lubalin Graph 72%
News Gothic 77%
Optima 69%
Times 68%
Univers 70%

To precisely determine the size for any font, use the line spacing gauge on this ruler to measure the cap height, and divide it by the cap height percentage of the specific font.
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Can a typographer help me to understand this please?

thanks