Originally Posted by Gravity Worx
Cutting the bottom of the frame head tube just moves the lowest point on the stack to where it fits.
Cutting the materiel from the top of the head tube effectively lowers the top point on the stack in relation to the rest of the frame.
When re-assembling the bike, there will be a slight difference in head angle if the cut is made on the top side of the head tube.
You were good right up to the point you started typing this.^ I am glad you acknowledged that the axle to crown height of the fork is fixed. Let's use the crown race (which rests atop the lower crown) as a fixed point of reference.
Now visualize if you will a bike sitting at rest. If you remove material from the bottom of the head tube the attitude of the frame will rotate forward around the rear axle. This will reduce the absolute height of the bottom of the head tube from the ground, shorten the wheelbase, and steepen the head angle. Got it?
Now visualize removing material from the top of the head tube. What will happen to the absolute height of the bottom of the head tube/crown race from the ground? That's right, absolutely nothing. Which, not coincidentally, is the exact amount of head angle change you will see.
<fingers crossed that you will get it this time around>
edited for clarity