Because of garage door height constraints, we have to remember two separate but important hitch heights. For the past couple years, I've used a tape measure, but that means wandering between the landing gear switch and the hitch, tape measure in hand. An easy solution is a length of yellow plastic chain, cut to the appropriate length.
Two key rings were installed in the appropriate links of the safety chain to assure that the measuring chain is connected to the correct links.
After the trailer is rolled out of the garage far enough to clear the forward escape hatch and canoe rack crossbar, the chain is re-positioned to the lower key ring and then the landing gear is extended until the chain hangs free.
I use a single "Hitchin' Rod" on the ball to ease alignment. It lines up with the centerline of the hitch structure.
For this trailer and hitch combination, 45 inches ground clearance gives about 1/2 inch ball clearance when the truck is in place. The Hitchin' Rod is stowed crosswise against one of the hitch plates. It was shortened to fit.
When the plastic measuring chain is not in use, it's stowed in a plastic food storage container mounted under the forward deck. I've used Lock N' Lock brand before, but this one was some other brand that was "gifted" from the kitchen.
Simple, cheap, and works well, the way mods are supposed to be!