Trying to get a mental picture of your method to find steering center. That does not seem to take into account the adjustability of the tie rods. Might you have a photo or two? In my mind, centered steering begins at the steering shaft and inner tie-rod ends, and goes out to the spindles via the connection, not the other way around.
The kingpin bolt is just that, and should be straight. All of the European (metric) karts used to employ 8mm. Now most have gone to 10mm. Your kart should have what are called “camber pills” attached above and below to the “C” shaped part of the frame where the spindle attaches via the kingpin bolt. Those items are what have the offset holes, and they can be rotated to adjust camber and caster. Note that adjusting the pills will also change the toe setting, so toe comes last.
Here was the first diagram of some camber pills I found:
There are many styles, and each manufacturer has their own way of approaching the engineering task. Recent TonyKarts have what looks like a flying saucer, with 20 perimeter holes to set the adjustments.
I am aware that some karts were delivered with non-adjustable kingpin placement. The kingpin merely rides in holes in the spindle-mount “C”. If I had one of those, I’d go straight out and modify the frame to accept camber pills. The only alternative is bending the frame where the spindle attaches to cure a camber issue.
BTW, all of what is being discussed is dependent on the frame itself being straight and square. It does not have to be perfect, that is what adjustments are for! I just got back a frame from being straightened at PKT in Wixom, MI. Looks great, Kim did an awesome job. Can’t wait to get it together and drive it!