If I got this correctly, the targeting circle values for Katyusha are under fatality_katyusha_rockets, and the radius of the reticule is 20.
Fatality is for the end of game fatality abilities that hit your opponents base.
abilities\soviet\modal_ability\artillery\artillery_barrage_ability\kaytusha_rocket_truck_barrage_mp
&&
abilities\soviet\modal_ability\artillery\artillery_barrage_ability\kaytusha_rocket_truck_barrage_vet3_mp
The regular barrage has an UI Radius from min 8.33 to 22.5 max (range 50-150)
Weapon stats:
Range min 50 max 150 (200 on weapon because it's also reused for the 200 range vet3 barrage)
Angle 10
Distance max 15
Ratio 0.1667
Angular scatter estimate as in first post: 5 - 15 (50*0.1 150*0.1)
Actual(?) angular scatter: 4.37 - 13 (distance/tan(90-angle/2))
Distance scatter: 8.33 - 15 (50*0.1667 150*0.1667 > 15)
So the Katyusha reticule overestimates angular (horizontal) scatter at all ranges. Error on distance scatter isn't as bad.
While testing this stuff it definitely feels like the explanation for offset in the mod tools is completely wrong. I always expected the hit area to be a half circle at 1 based on that, but instead it seems the offset just moves the entire possible hit area forward (or backward with a negative value). With a value of 1 all targeting is now centered around what used to be the edge of the old scatter area.
Suxton and its 0.3 offset after around 40 rounds or so:
Light red circle is ingame UI reticule circle. Light green ellipsoid part is calculated scatter centered around the targeting circle. Blue ellipsoid overlayed on top is an offset moving the entire targeting area forward. Ignoring height differences, no rounds will ever hit in the light green or light red areas, everything will hit in the area overlayed by blue (so dark green+blue+purple areas).