Byron writes, “Is there an advantage in measuring in points or picas over inches, and if so, what is it? Also, where do you buy the best steel or other ruler to do it?”
Inches are best for U.S. paper sizes.
Picas and points are the traditional measure and what I learned. There are six picas per inch, 12 points per pica, 72 points per inch, all easily divisible, unlike inches! A point is identical to a Mac pixel — the Mac runs on 72 pixels per inch — which makes paper-to-web transitions easy.
U.S. type sizes are measured in points.
A “computer” or “PostScript” pica is rounded to exactly 1/6 inch. An old-fashioned, traditional pica is .000622 inch shorter than this, so physical rulers no longer work. I’m unaware of any physical pica rulers (also called pica poles) that use the modern, computer pica.
I make my own rulers with inches on one edge, picas on the other, hairline rules. Depending on your printer, you may need to adjust the output size, usually in fractions of a percent, until it matches a real inch ruler exactly, and the picas and points will be correct, too. Also works for millimeters. Print on heavy stock.
Here for download is the file I made for myself.