Your illustration is a classic, and I like the way you further refine it on your site. You provide a lead that is worth following.
It doesn't provide as much information, but I try to accomplish the same goal using a 2x11 strip of paper laid diagonally over the important subject. With the strip behind my back, I turn the focus switch so I can see where I have to put the strip. Then I turn off the enlarger and drop the strip where it needs to be.
I make four or five slivers by moving a piece of cardboard about a half-inch in "third-f/stop" increments. I'm most satisfied when I see it go from "too dark" to "too light".