If you average all gunsights used in WW2 fighters their common factor is a 100Mil ring projected on the reflector plate set 16.5-17 inches from the pilot's eye for the best angle of view. This will average a 2inch diameter projected ring. I checked a few fighters measuring a 100Mil ring from default head position, half zoom and full zoom. P51D, La7, Spit16 and P40N. P40N has an N-3 gunsight which the maximum ring was 70Mil.
100Mil ring.
Default = 5/8 inch diameter.
Half Zoom = 2 inch diameter.
Full zoom = 4 inch diamter.
70Mil ring.
Default = 1/2 inch diameter.
Half Zoom = 1 1/2 inch diameter.
Full Zoom = 2 3/4 inch diamter.
As for the default view and moving it. The main ring on the gunsight has been set to remain a fixed diameter no matter how far back or close you move the default head position. I don't know Hitechs reference distance 16.5, 17 or 18 inches. You can make the ring or aimpoints in your gunsight appere scaled larger or smaller on the reflector plate by moving the default position forwrard and back. But, the default distance between your eyeball and the ring will not change how ever far forward or back you arrow key the default PoV.
Your arrow forward motion for every fighter is limited to the vertical aft members of the windscreen assembly. In F3 mode the P40E/F/N has a very deep windscreen area with the gunsight forward into it. The P40C has a shallower windscreen structure allowing more travel forward to the reflector plate. The La7 has a shallow windscreen area and the gunsight support arm holds it towards the pilot.
I beleive you are describing the specific demension differences between fighter cockpit windscreen designs and structure relative to how gunsights are mounted to the pilots default head position.