Originally Posted by
oneclick
You are over-thinking this (and perhaps so are some others).
Take the crank off and make sure the spindle is correctly adjusted.
Mount the crank without any rings. Turn it once, measuring how far each arm is from the chainstay. If they are all the same it then mount each ring alone, and measure them the same way.
If arms are not the same bend them until they are - bend them on the bike, you can check your work each time right away.
If the rimgs are not true they are best bent unmounted, with a flat shiny surface to check against - a mirror or a bit of granite countertop will be flat enough, though if you have a surface plate by all means use it.
Assuming the cotter is properly pressed and isn't throwing the arm slightly off-center, this is absolutely the best approach - same general path to checking dropoout hanger alignment. Remove rings, mount and properly press crank cotter in. Measure each of the three spider arms for distance off the seat tube at peak turn. Large adjustible wrench to get them all in sync with each other. Re-mount rings.
Big ring usually isn't swaged on these three-arm cottered cranks from Solida and the Nervarr cranks too.. The three-arm spider typically is swaged, large ring held on by three bolts, small ring held to large ring by three bolts.