My vintage English 3-speed has a chainline issue. I finally traced it to the RH chainstay being pushed over to the right side. Using Sheldon's string method described here

Bicycle Frame/Hub Spacing

I determined that the difference between right and left with respect to the seat tube is about 8mm. (I ran fishing line around the head tube, tied it off to both rear fork ends, and used a ruler to measure the difference between the seat tube and the string on both sides. The right side was +8mm more than the left. Incidentally, the rear clearance is around 118 mm--a bit wide for the standard 110 mm spacing on these old hubs. Pushing the right stay 8mm inward would both close this gap and fix the alignment issue).

I called the LBS, and upon mentioning "frame misaligned" they declined to do the work. I did locate a guy who is in town and is a framebuilder (and has a frame alignment table), but he mentioned that I would need to install a bottom bracket insert. I'm not sure if these inserts will fit my bottom bracket or not. The bike is an old English 3-speed with cottered cranks.

So should I attempt the Sheldon method on my garage floor. Are there any improvements to the method that I should try? The method seems pretty crude, but I do trust Sheldon's wealth of knowledge. The frame is lugged steel, and is pretty mild--unlike more modern steels.