Source: https://github.com/liurui39660/PayDay2.Mod/tree/MoreADSZoom

Change the global zoom level.
It should be applied to all sights, including gadget sights.
Put the decompressed folder into mod/ folder, like most other mods.

Tl;dr, for the zooms, vanilla < scheme B < scheme A; for ADS FOV, the order is reversed.

If you don't know how sights work, you can read this guide. There's a section (almost) comprehensively explains the mechanism about sights. One thing the guide seems to forget to mention is gadget sights are not +X sights; they override the zoom levels.

I use the hip-fire FOV 65 as the baseline (1.0x); when giving examples, I assume weapons' base zoom level is 1 (typical sniper rifles).

This mod provides two schemes, scheme A has 10 levels. FOVs range from 1.25x to 8x. This is also the only choice in the initial release of this mod. These levels are

63 -> 63.00, +0, e.g. iron sight, no change
60 -> 52.00, +1, only marksman sight and bipod, 1.25x
55 -> 43.33, +2, only surgeon sight, 1.5x
50 -> 32.50, +3, e.g. red dot, 2x
45 -> 21.67, +4, e.g. milspec, 3x
40 -> 16.25, +5, e.g. acog, 4x
35 -> 13.00, +6, default sniper scope, 5x
30 -> 10.83, +7, broomstick scope, 6x
25 -> 9.296, empty level, 7x
20 -> 8.125, +10, theia, box buddy, a5 sight, 8x

In 2.0.0, scheme B is also provided. This scheme only has 8 levels, FOVs range from 1.25x to 6x. The purpose of this scheme is to 1) allow high-level scopes useable in medium-size maps and 2) allow players to see the on-sight range finder. The zoom levels are

63 -> 63.00, +0, e.g. iron sight, no change
60 -> 52.00, +1, only marksman sight and bipod, 1.25x
55 -> 43.33, +2, only surgeon sight, 1.5x
50 -> 32.50, +3, e.g. red dot, 2x
45 -> 26.00, +4, e.g. milspec, 2.5x
40 -> 21.67, +5, e.g. acog, 3x
35 -> 16.25, +6, default sniper scope, 4x
30 -> 10.83, +7, broomstick scope, theia, box buddy, a5 sight, 6x

The preview screenshots are respectively Theia Magnified Scope (+10), Advanced Combat Sight (+5), Speculator Sight (+3), Milspec Scope (+4), using the scheme A, with the mod on/off.

Comments and suggestions are welcomed.

Demo
All uses scheme A.

For Theia, in a random forest ↓

For Advanced Combat Sight, in someone's breakout ↓