1x (rds): fov 40, speed 0.5: 80 ratio
1x (irons): fov 40, speed 0.5: 80 ratio
holo: fov 32, 0.32 speed: 100 ratio
3.4x: fov 20, 0.36 speed: 55.5556 ratio
4x: 17.2000001, 0.31 speed: 55.4838 ratio
6x: fov 11.6, 0.21 speed: 55.2380 ratio
7x: fov 9.9, 0.21 speed: 47.14 ratio
8x: fov 8.7, 0.16 speed: 54.37 ratio
12x: fov 5.8, 0.11 speed: 52.7272 ratio
the "speed" is what you want to change, factor-wise.
so if you've been using holo (speed 0,32), and want to go to irons/rds (speed 0,5), you want to increase your sensitivity by factor 0,5/0,32 = 1,5625
So: 0.0475*1,5625 = 0,07421875