The most important thing regarding axle alignment is camber, which is generally built in when the axle is built...
Then that all trailer wheels run true to one another....(critical)
Then the measurement from the ball socket to the spindle center. This measurement is not near as critical as the first two, the longer the trailer the more this is so.
This is just something I have picked up over many years of dealing with lots of trailers.
1/2 Ton 4WD Truck, 72 Sovereign