For the first time I find myself disagreeing with you 18.
In my experience sights are measured from the bottom of the dovetail to the hightest point of the blade. I'm sure there are exceptions, but that is the norm.
Now before you buy a sight, are you shooting low/left? Are you a right-handed shooter? If that is the case, then you don't need a different sight. What's happening is that when you squeeze the trigger, all your other fingers are also squeezing, and those fingers are pulling the pistol down to the left.
If you're a lefty, never mind.
Cat