You have to move the 'dirty' air coming off the windscreen up over your helmet, with a taller windshield, or down below your helmet (with a shorter windshield). That said, one critical variable is how 'tall' you sit in the saddle. The height variable is largely responsible for the fact that what works for one guy (windshield height-wise) doesn't work for another.
When you're riding down the road at cursing speed, take your left hand and move it up and down off to the side near your face shield to determine at what height relative to your helmet the wind feels 'smooth', and where the wind feels 'roughest'. (It's the 'dirty' air striking your helmet that causes most of the buffeting).
Where the wind is roughest is about at the 'center' of the dirty air. You want to move this 'center' above your helmet top, or below its bottom. Decide if you want to look through the windshield (move the dirty air up) or over the windshield (move the dirty air lower).
Gauge how many inches it will require (based on your 'hand measurement') to do this, and that will give you an approximation of how much taller, or shorter, the new windshield needs to be compared to the one currently on the bike. (This assumes you buy a windshield of the same shape as the one you currently have, BTW.)
Another variable is helmet shape. Some helmets will 'buffet' more than others in a given amount of rough air. Also, you might not have to get all the 'dirty' air off the helmet, just the worst of it (what was called the 'center' above).
My 2014 with a 7" Windsplitter works pretty well for me. If I go directly into a hard wind (30-40 knots or more) it adds to the buffet and I can feel it. That is, if I'm running 70 MPH, an additional 40 MPH head-wind makes it like I'm running 110 MPH. (So speed has something to do with the 'buffet equation' as well.) But on a normal (less windy) day, I get some dirty air at the top inch or so of my helmet, but it's not enough to bother me.
HD has a 'try before you buy' windshield program, so you can experiment at no cost.
alan