Do they measure to the top of the hair at the shoulder or do they mash down to the top of the shoulder?
I used to use a small hardcover book dog standing square, measure at the withers right where the neck and shoulder meet. You will feel a bone on the top that is the withers, place the book there don't push just place. Measure from the floor to the bottom of the book and that is your height. I found doing it against a wall worked best. You will probably need someone to help you hold something.
Is this for Agility? There is a good YouTube video of Jackie Gardner demonstrating the KC official method for measuring dogs for Agility. Titled, How to Measure Your Dog for Agility.