I’m not sure you understood my “simplified example” question so I’ll restate things a different way.
Factory Service Manual says my toe should be 3/16”.
The link tclederman shared wants me to subtract the distance between the front tire tread centerlines from the distance between the rear of the tire tread centerlines (B minus A).
I’ve been thinking if I cause my tires to “toe in” 3/16”, then the difference between the front and rear will be 3/8”. If I do the math I get 65.375” - 65” = 0.375” but in reality the tires have only moved toward each other by 0.1875” (which is 3/16”).
Not the way we've always done it. If the spec calls for 1/8" toe in, and, let's say, we're working with a roughly 61" tire center line to tire center line, or "A" and "B" as the diagram shows, then your B measurement will 61" and your A measurement will be 60 7/8". A difference of an 1/8" from front to rear. Thing is, you might've only moved the front or rear 1/64" to get the number you need, if even that.
One quick way we would get a reference mark is to raise the front of the vehicle. Using white or silver paint, spin the wheel while spraying the center of the tire. Let dry a bit. Spin the assembly and hold a scribe to the center of the paint to make a reference mark. Eventually, after a little driving, the paint comes off. These days, some new tires already have the reference mark from the manufacturer.