I can see 20 thou..8 would be very nice..
When truing a wheel, i use a marker or tape to mark certain areas.
I would mark the weld, and it you are using a dial, lift over the weld or just ignore it.. Ie measure just before and just after the weld..
If the deviation is short, ie weld width, the bead of the tire will not distort much, and it will probably not be noticeable..