The absolute best way to measure a draw length, is to do it while shooting. Mark an arrow with some rings of varying color every 1/2". Have an observer with a keen eye, watch you shoot a dozen or so arrows and take note of each shot. It's best if they don't tell you what they are seeing at the time, so you can just focus on your target, and so you don't try to reach any particular draw length. If you have a camera with a burst mode, all the better.
I had my wife take some pictures of me a couple of weeks ago. It's hard to tell in that picture, but there are rings around the shaft at 30, 30.5, and 31". The 31" ring is right above my index finger and the 30.5" is just hidden by the riser. Again, this was taken while I was focused on hitting my target and not posing for a picture. The rings on the shaft were almost gone from wear, and I wasn't using this particular photo session to determine my draw length, but you get the idea.
A word of warning, though. If you post a picture of yourself, a whole clan of yahoos will commence to screwing up your form
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Oh, and don't shoot a bow with your favorite mechanical pencil in your shirt pocket. It's amazing just how far one will fly if the bowstring hooks onto the clip.