You want the height from the waterline to the highest point of the spar? (As though to pass under a bridge?) A guess within inches would require knowing how your boat is rigged; i.e., "Geezer-rig" or "Racer-rig".
Yes, L&VW's post is spot on. The height from water to the tip of the upper boom (gaff) will differ depending on how low or high on the spar the halyard is attached. If this s a bridge clearance issue, and you sail in tidal water, also don't forget that clearance is also based on high or low tide.....a lesson this freshwater sailor learned many years ago in the Florida Keys.