I have done a simple task and it has worked for me all the time..... since I watched a movie.
I will place something that I know what size it is, for example, a credit card, a 6" squeegee or bigger, or the one that works the best, a ruler, tape to the door and take a picture, then I import that into my illustrator app and enlarge it to the size of the ruler. you will be off about .25" to .5". Hope this helps if you call your customer.