Have you thought about changing up your front sight to the correct height? Ameriglo has a sight calculator for Glocks and they can help you find out the exact height you might need for the ammo you shoot. Check this website out for guidance.
http://www.ameriglo.net/sights/calculator
This being said I do now know the answer to the OP's question about where the impact is suppose to be. I just change my sights so the impact is always at the top of the front sight. By using this calculator you could just crunch the numbers to where YOU want the impacts to be (whether its top notch, center of dot or 6 o clock). Just plug in the deviation you want and do the math!
I always recommend this little nifty tool for glocks....
http://www.10-8performance.com/produ...t-In-Tool.html. Its cheaper than buying a front sight then another one. Just file it down with a gunsmithing file until you get the correct height for you. Then measure it with calipers, then order that front sight height from your favorite manufacturer. Saves money in the short run, much less the long run if you buy a couple of them.