One can vary how much comes off the FA with a stiffer rear spring set. Having the rear set a bit higher so it takes more drop of rear to make the frame level to going downward so you start removing weight from the front axle.
I noticed this going from 6400 to 8400 lb springs. Weight taken off front went from 350-400 lbs to 250-300 lbs with a 1500 hw trailer.
I've noticed anything over around 400 lbs off of the 8 lug SW trucks I've had, it helps to have WD bars installed from a handling prospective. You are not required legally to have any bars towing. Only fro a warranty standpoint.
Marty.