I'd be going with a BoF estimation rather than "winging" it by guessing somewhere between the lines.
Here's a link to a previous thread where we discussed this stuff and I posted my method: http://forums.epicski.com/showthread...&highlight=bof
Note that the hardest part of the method is determining where the ball of your foot falls in the boot shell when you're buckled in - that's why this method is not 100% precise. There have also been some discussions that modern skis (coupled with modern technique) are driven more from the arch of the foot rather than the ball, thus making the whole BoF method less applicable. My experience with fore/aft positioning adjustments has been very positive though.
The main issue is that depending on your foot size the distance between the ball of your foot and the midpoint of your foot will vary. People with larger feet will naturally have the ball of their foot further forward on a ski than people with small feet (if they both mounted at the marked midsole point).
I have moved bindings both forward and rearward to "dial in" the ski's performance. This does so much for changing the character of a ski that I now mount plates on almost all of my skis to allow easier adjustment of the bindings both fore and aft.