Doesn't measuring from the rear wheel spindle (axle) to the pivot axis of the rear swing arm assume the front wheel is centrally located with respect to these same rear pivot axis?
Put another way, doesn't it assume the steering head bearings, triple clamp, front forks and the central location of the front wheel to the forks are all correct and centrally located with respect to the rear pivot arm bushings?
I'm not sure I can make that assumption on my G5.
That's why I use a long straight aluminum pole resting against the sides of the rear wheel and extending along side the front wheel to check my rear wheel's alignment.
(Yes, I have to snake it thru the centerstand which is why I can only use the lower part of the rear wheel to establish the wheels direction.)
I might also add that I do this check on both the left and the right hand sides of the wheel.
When everything is aligned, the pole just clears the narrower front tire.
Anyway, back to the snails.
I think the reason for the large difference in the snail location is due to the inaccuracy's in welding the small stop pins into the trailing arms.
My snails have a 1 notch difference so a difference of 4 does sound like a lot to me.
If mine had that large of a difference I would be tempted to remove my hands from the handlebars briefly while riding down a smooth flat road.
If the bike didn't decide to go left or right I would know it's just the nature of a "hand built machine".