On a flat and level surface, with nothing hooked up, the top of the trailer hitch ball pocket (on the level trailer) should be the same height as the top of the trailer hitch ball (on the level tow vehicle). You adjust tow vehicle ball height to accomplish this as the trailer ball pocket height is non-adjustable.
Obviously, after making these measurements and settings you will have to raise the trailer tongue (unleveling the trailer) enough to get the hitch ball under the hitch.
After connecting, other factors like tongue weight and spring bar tension will come into play. However, start out with trailer and tow vehicle equal.
Dealers seldom set up a trailer hitch properly.