the 100 MIL sight was used, in RL, to aid gunnery.
the proper lead 90 deg deflection shot for a target moving at 100 MPH was 1 radius of the 100 MIL sight.
2 radi for one moving 200 MPH,
3 radi for one moving 300 MPH,
4 radi for one moving 400 MPH and so on
using the 'Rule of Thumbs' for the sight and dif lead angles was also simple
for 60 to 90 deg of deflection you use the full deflection
for 30 to 60 deg use 3/4 of full deflection
for 15 to 30 deg use 1/2 of full deflection
for 0 to 15 deg use 1/4 of full deflection
the guys that had to do this for real were pretty smart

its usefullness is why Im looking for a true 100 MIL sight