George, I don't want to get too far off topic, but...
Where did you find that image in your post above?
I've always been under the assumption that the imaginary red line joined two points: the center of the rocker arm trunnion, and the center of the roller tip trunnion. Hopefully these images convey my meaning:
I've always used Jim Miller's methods to set up various engines:
http://www.sbintl.com/tech_lib...ker_arm_geometry.pdf