Interesting - that's significantly off from my numbers (which are, to be fair, semi-educated WAGs). I ended up with an arm of 3.7" for either a Hartzell metal or Whirlwind prop. To get these numbers, I started with an estimated prop flange arm of 8.3", and then assumed (as you did), that the CG of the prop would be at about the midpoint of the blades. I used dimensional drawings from the prop manufacturers to get an estimate of prop arm relative to the flange, and then summed the two numbers to get the final arm relative to the datum stated in the construction manual.
My best guess here is that my estimated prop flange location is way off. I don't recall where I got that number from, though - I did a whole ton of searching while trying to put this together and it may be that I just took someone's word for it. Based on Paul's measurement and my estimation of prop CG relative to the flange, seems like ~16.6" is a more accurate estimate of the flange location.
I guess it's a good thing I'm only using my little utility for "advisory purposes," as the phrasing goes.