I'm not sure if this is needed because I'm not sure anyone but myself makes gunsights in 512x512 where the mil file is needed to control the view size of the gunsight in the cockpit.
Some of the historic gunsights I make don't fit the default head position gunsight glass size. You can move your FoV position forward. But, that eats into the peripheral side views. The PBP1 Late and San Giorgio tipo C full sized gunsights come to mind. Or the Grumman and F4u family needs the gunsight increased in size from 256 to 320-384.
My thought is having a slider added to the gunsight application that takes the place of the mil file in which you can use numbers below and above 256 to decrease or increase the viewable size of the gunsight if it is made on a 512x512 mask. I'm pretty sure most players don't take advantage of what a 100Mil ring means to estimating lead. But, would prefer a good looking cockpit with the gunsight fitting nicely.
Below are two screen captures to illustrate this in the C.205 at the default forward view head position. The main ring of the tipo C is 100Mil. But, it has two lead arcs that are part of a 120Mil ring for a 30 degree hold off. The first picture is the gunsight at default sizeing using 256 in the mil file. The second is reduced by 32 to 224 which fits the gunsight to the reflector plate.
Tipo C at full size, 256 in the mil file. 
Tipo C at reduced size, 224 in the mil file.
