I just got done doing a little "homework" on this end to show you exactly how this works..

AP is working on a Lefty fork.
Lefty fork has both an upper and lower clamp.
She is contemplating cutting the head tube to make room for a head set.

There is both an upper and lower clamp.
Head tube with headset essentially needs to be shorter to fit.
Cutting the top side will allow the frame's head tube to sit that little bit higher between the 2 clamps or crowns. Gets more slack of an angle.
Maybe 1/4 degree difference or so, but still a slight change none the less.
Cutting the bottom in the 2 clamp systems such as your lefty will make room, but will not change head angle.

Now on a single clamp or crown fork, just the opposite is true because it's lined up with the lower and not the upper.
Therefore when cutting the top side of a single crown or clamp fork, nothing happens but a shorter overall headtube/stack height.
But when cutting the bottom side on a single crown fork, the frame lowers on the fork slightly and thus, affects head angle slightly, Lowering the front end makes it more steep.
Again, only a slight difference, but still a difference.

The fork height does not change.
The headset bearings will obviously sit at the max height that they can or she would not be looking at cutting the frame.
So it's back to cutting the head tube down to make it fit the given space.
The head tube is going to be as high up in the fork as possible as there is no room for anything different.

So this boils down to cutting materiel off the top and effectively raising the frame head tube that 1/4" or so, or cutting the materiel off of the bottom and leaving the top of the frame at the same height it was.
I drew a quick picture and attached it. I know I'm not an artist, but it does show what I'm explaining on how it will effect head angle slightly.