Currently we have very simple system of sight adjustment which is OK for ironsights but when it comes to sniper scopes things become more complicated.

My suggestion is to add MOA or Milliradians adjustment and use the current 100,200,300m etc. as a BDC on the adjustment turrets.

Currently we have the next system:
When you press PgUp or PgDown you move up or down in the next list
1000m, 900m, 800m, 700m, 600m, 500m, 400m, 300m, 200m, 100m

The system I suggest works with a little addition (ACE .308 adjustment tables used as a reference):

Main adjustments are MOA/Mil but some of them has BDC markers. That means that by pressing PgUp and PgDown you choose among the BDC-marked values and set corresponding MOA/Mil value;

for example if your current adjustment value is 32moa which matches 900m and you press PgUp you jump to 1000m and set 41moa, and if you press Up you will have 42moa adjustment and so on. If your adjustment is set not to marked position (for example you have 12moa on your scope at the moment) pressing PgUp or PgDown will lead to setting to the closest BDC-marked adjustments – 14moa (matches 600m) and 9moa(matches 500m) respectively.
What’s seen at GUI Fast BDC adjustments (markers) MOA/Mil adjustment (press Up or Down)

Fast BDC adjustments (markers); MOA/Mil adjustment (press Up or Down)
1000m; 41 moa
--; 33-40 moa
900m; 32 moa
--; 25 -31 moa
800m; 24 moa
--; 20 -23 moa
700m; 19 moa
--; 15-18 moa
600m; 14 moa
--; 10-13 moa
500m; 9 moa
--; 5-8 moa
400m; 4 moa
--; 1-3 moa
300m; 0
--; -2 -- -1 moa
200m; -3 moa
--; -6 -- -4 moa
100m; -6 moa and less

This system requires fast BDC-adjustments to be connected with the gun itself not with the scope (the scope only has its MOA/Mil adkustment range, the BDC-marks depend on the gun itself). Probably this connection has to be made somewhere in gun's config (probably based on the ammo that weapon uses to make it more realistic and dependable on ammo, for example subsonic ammo has very different ballistics comparing to ussual rounds).