Tap every state this load runs in; the strictest rule wins. Nothing tapped means no kingpin check. These rules only cover trailers over 48′. Other states aren't loaded yet.
Change the hole after you slide and the kingpin distance moves with it. The kingpin distance is the one number worth calibrating (see below); you can read the rest off the trailer. 12,000 is a common steer rating, not a federal limit.
Weigh, slide a known number of holes, and weigh again. The two tandem readings give the kingpin distance exactly, no tape measure needed. Enter the second ticket with the pins where they are now.