Well yeah, if the kingpin is off center that will mess up measurements. Not likely to be off & splitting hairs anyway.
Easy enough to measure. On level ground drop a plumb bob from the kingpin. Mark or pound a nail into the ground & measure to an easy point on the axles. U bolts will do so long as the same position U bolt each side & fwd & aft are used.
If one axle is straight adjust the other. I installed the Correct Track on mine. Easy enough to do. One side at a time. All there on YouTube. Toughest part is getting the hanger bolts back in.