In my experience with doing optics for SH3, I have found that the best method to determine true degrees dimensions in the game is actually already built-in, and that is the hardcoded german reticle (Which is in degrees). I recommend you to fire up the game in the highest X=*/768 resolution (4:3) your monitor can handle, and then choose a german uboat and look through the periscope. Take a screenshot and count the pixels between each degree mark, and you will have a very good indicator of that dimension in the game. Then you can adjust the periscope mask and reticle markings in size to ensure it has the correct FOV in terms of linear horizontal pixels. It will work directly for the US counterpart, as the cameras.dat file is shared
Also, don't take that 1x/4x vs. 1.5X/6x thing to seriously. In game terms, augmentation is only equivalent to field of view, and hence you can have a 1x periscope that actually produces the correct linear FOV for a 1.5x. This is because you have in the game two values to play with: Augmentation and Camera aperture (FOV), whereas in real life you don't (They operate in different areas). But the end result in the game is the same, because true augmentation is not possible to be scaled accurately, as you have different sizes of monitors for all players, and you look at them from different distances. The important thing is that between both zooms the difference is 4x, because specially in US optics the small/large marks come in series of 4, as it was primarily designed with 6x in mind, where each big mark is 1 degree and each small one 1/4 degree.
Hope that can give you some ideas, and excuse if you had thought of this all already
