Like Uncle Bob says, you've got the kingpin center to center dimension plus the wheel offset dimension that sets the centerline of the tire relative to the frame and fender. If you're running fenderless (your profile says Rat Rod) you've only got the frame affecting the turn radius to worry with. The other dimension is your spring perch spread, and that needs to be coordinated to the spring to give you bounce without binding.

Welcome to CHR, BTW!