The professional alignment mechanic (as I was in a former life) has not the time for doing needless math when the spirits level for caster does all the work for you. It is a correct statement that one interpolates the caster from a camber change while turning the wheel through 40 degrees. You turn the table 20 degrees in, rotate the gauge to level it side-to-side, then turn the thumbscrew to center the caster bubble. Then turn the table to 20 degrees out, rotate the gauge to level it side-to-side, and read the caster bubble.
I can't help you with the 1.5 times thing, because, as you can see from above, it's a snap with the proper tools. Nobody earning a living is gonna take the time.
As for the theory of caster, although it might matter on how it gets adjusted (or IF it's adjustable at all), the nature of the front suspension matters not. There is always an axis around which the front wheels steer. If there are a pair of ball joints, it's the imaginary line that passes through the center of the upper and lower ball joints. If it's a strut, it's the imaginary line that passes through the center of the upper plate [where the strut rotates] and the lower ball joint. If it's a kingpin, it's the imaginary line through the center of the kingpin. If this imaginary line intersects the ground FORWARD of a vertical centerline through the spindle, that is positive caster. If it intersects the ground rearward of the spindle centerline, that's negative caster. Some people mistakenly oversimplify and visualize the imaginary line tilting BACK for positive and FORWARD for negative caster. In the vast majority of automotive applications, that works, but it is technically the point at which the imaginary line intersects the ground. The "casters" on your rollaway or shopping cart "seek" positive caster as the wheel trails behind the pivot point. That pivot point intersects the ground FORWARD of the vertical centerline of the wheel's spindle, and it has nothing to do with the "tilt" of anything.
Back to the 1.5 thing, it might be easier (and more accurate) to obtain an inexpensive alignment gauge. Otherwise, sorry I can't help there, and good luck.
