This is how I have read to do it. I have not measured my pups height yet.
You’ll need a level surface adjoining a blank wall, a carpenter’s level, a pencil and a metal measuring tape or a yardstick. You also need a helper unless your dog is perfectly trained to hold a stand-stay. Have your helper position the dog so it is in the above position, with the dog's head level. Place the carpenter’s level across the withers, letting it rest on the bone, one end at the wall. Move the ends of the level until the bubble is exactly in the center of the level indicator, then use your pencil to mark the wall underneath the level where it touches the wall. Move your dog away from the wall and measure the distance from the mark to the floor. If you have been careful - and honest - you now have an accurate measurement of your dog’s height.