How to determine the correct width for a Roman upper case slanted stroke, per Mr. Claflin, 1922?

Hello,
 
I'm working through a project of drawing and inking all of the Roman upper case letters, using the instructions given in the 1922 edition of "Standard Lettering" by Roy C. Claflin.

Since this is a textbook from the Columbia School of Drafting, I find most of the instructions to be very clear, concise, and businesslike, except this one about measuring the width of the slanted, "heavy stroke or stem" of the Z (and N, M, W, V, A, etc.): 

• "Rule 5. In laying off the heavy stroke or the stem of the "Z," its width--one unit--must be laid off at right angles. This applies to the heavy stroke of all letters. This is done by measuring from the upper right hand corner of the "Z" at right angles to what you can easily estimate will be the slant of the heavy stroke. Connect the point just laid off with the lower left corner of the letter and draw the other side of the heavy stroke parallel to the first line."

Some elements of this (for example, what constitutes a "unit") have been previously established by drawing the I, H, L, and T. I just can't decipher where the "right angle" Mr. Claflin is referring to is actually measured. 

Can anyone help me "see" what is being called for here, please? It's stopping progress, since about 8 of the next 10 letters require this measurement!

Thanks in advance!