The picture here is straight out of the Springfield Armory manual for my XD-40.
When aligning the sites as per the picture above, I'm putting the rounds into the dirt or at least several inches below where I want them to go. I've learned that in order to hit the targets where I want to, I've had to make a little triangle out of the sites. I place the front site a little above the horizontal line made by the two rear site posts. I can at least hit the broad side of a barn like that. Just ask Buckfever about that point!
Oh, I had a qualified armorer put tritium (glow in the dark) sites on my XD a year or so back, but remember having the same issue with the 'plain' sites before that. Anyone know what happened to either me and/or my XD???