The height is determined by tow vehicle suspension, trailer tongue weight which varies due to what and where you load personal items in the trailer. There is no specific height. It varies for every situation. Best advice is to hook trailer and adjust ball height and WD torsion bars for a level ride. This should be done at pickup by the specialist who performs the pickup checklist during your walk through.
2011 Chevy 1500 SB 4x4 5.3L w/trailer tow
2013 Zinger ZT23FB
Reese Weight Dist. w/friction sway control