Everyone is knit together differently. Some tall people like short LOP, some long. I'm 6'2" and started out very comfortable at 14" and now 15 years later I shoot about 15". There is no ballpark estimate because you are a unique individual.
No in fact tall people often need very shot lenght of pull if they have a short neck
The neck length is the most important measurement
LOP however is not very important unless it is way too short (you hit your nose when you shoot) or way too long (you head is pulled off of or moves on the stock when you shoot) most people have about a 1 to 1 1/2 inch range there
mounting a gun in a static position tells little if anything because you will try to adjust your body to the stock-- all that goes out the window when you move and shoot in a dynamic way-- dynamic movement
Try a couple of shotguns at your club and see what LOP suits you. This can be particularly helpful if you can find a gun with an adjustable stock that the owner will let you adjust. It's even more helpful if the gun you try is the same make and model as the one you want to buy.
Hopefully, you'll find two or three guns with the same or similar LOP that feel good to you. Now compare that LOP to the LOP on the gun you want to buy. If the LOP on the gun you want to buy is close to the LOP you like, you can figure that you can adjust the LOP of the new gun to suit you after you shoot it for a while.
Yes, I understand that the gun you are looking at doesn't have an adjustable LOP, but, assuming the LOP of the new gun is relatively close to what you think you like, you can adjust it anyway by changing the thickness of the recoil pad, adding a black plastic shim or even by cutting off a small amount from the butt of the stock. The trick is to get a gun with a stock that is relatively close to the length that suits you. Also, a gun that's too long is easier to adjust this way than one that's too short - it's easier to cut wood off than to shape and add shims, etc.
Keep in mind that as you shoot the new gun, your LOP preference may change. So getting it "spot on" at the beginning probably won't get you home free anyway.