There's no need for the trial and error that has been suggested so far. There are clearly published specs:
Here they are:
A = Freewheel Stop to End-of-Locknut
B = Drive-Side-Flange-Center to End-of-Locknut
C = Outside-of-Locknut to Outside-of-Locknut
Regular 5 and 6 Speed
A = 29.00mm for 5 Speed
A = 35.00mm for 6 Speed
C = 120.00mm-122.00mm
Narrow 6 Speed, 7 Speed and 8 Speed
A = 31.00mm for Narrow 6 Speed
A = 36.00mm for Narrow 7 Speed
C = 125.00mm to 127.00mm
A = 40.50mm for Narrow 8 Speed
C = 130.00mm or 135.00mm
Most folks who want to build a classic 80s 126mm 6/7 Speed rear hub will simply go:
A = 36.00mm
B = 43.00mm to 44.00mm
C = 126.00mm to 127.00mm
IMPORTANT NOTE:
Get the freewheel side perfect as possible. That side must be correct.
Hold the hub and ruler "vertically" when reading your results to get a good read.
Use the non-drive side as your "buffer", "fudge" or "play loose" side in order to get "C". "C" doesn't have to be perfect. Most of my 6/7 Speed rears end up being 126.50mm when I'm done.
=8-)
__________________
5000+ wheels built since 1984...
Disclaimer:
1. I do not claim to be an expert in bicycle mechanics despite my experience.
2. I like anyone will comment in other areas.
3. I do not own the preexisting concepts of DISH and ERD.
4. I will provide information as I always have to others that I believe will help them protect themselves from unscrupulous mechanics.
5. My all time favorite book is:
Kahane, Howard. Logic and Contemporary Rhetoric: The Use of Reason in Everyday Life