Great thread - I know there have been similar ones over the years. Um, your methodology is OK, as long as you are open to admitting all the factors that will cause an error in the measurement. Believe it or not, the BIGGEST problem with your methodology is that you assign a definite measuring stick - this girl's hand, whose dimensions are only an estimate.
Think of it this way: What if I told you that someone was standing 4 car lengths away, but instead of telling which kind of car you were using as a measuring stick, you just found an average car length to use as a guide. Well how far away is he and how much error is in your guess? Well if the average car is 18 feet plus or minus 7 feet that means the person standing 4 car-lengths away can be anywhere from 44 feet away all the way to 100 feet away, a difference of 56 feet!!! But your guess is that he is 72 feet away, plus or minus 28 feet.
Does the girl in the pic really have average hands? I don't know. There are some pornstars out there that have unbelievably tiny hands. Tweety Valentine comes to mind: