I'd put a level on the floor in the trailer, running the level fore and aft and then level the trailer to that, then measure to the top of the coupler. Some people put a small level on the fridge
floor and level to that.
The tow vehicle will probably settle an inch or so in the back after hooking up so maybe add an inch to the tongue height as a target for the undeflected hitch ball height on the tow. Tongue weight
for 17s are around 300 pounds. Find a couple or 150 pound people to sit on the rear bumper to see where your vehicle's ball height will end up when towing. It doesn't have to be leveled to fractions of an inch.
My first thought was I wonder if you have the high lift option. Best to measure your own situation and eliminate the guesswork. Different tow vehicles will settle different amounts so using other's measurement might not help much.
But you said you can't get an accurate measurement where the trailer is now so my response is probably more useful to other new owners.
My picture (I don't have the high lift axle
, wish I did) should have the yardstick an inch or two forward but I couldn't get it to stand upright on it's own. But it gives you the idea. Mostly I was checking clearances before installing a "one-arm" WDH (The one arm swings under the jack post during turns to the right and I wanted to be sure it didn't catch on anything. That's why I used a scissors jack to hold up the tongue and raised the jack post to its top-most position. Just using the tongue jack is sufficient otherwise.)