What calculation is used to determine the deflection correction if the FO/SL is beyond 100m from the gun?

The calculation for determining the deflection correction when the forward observer (FO) or the spotting line (SL) is beyond 100 meters from the gun involves using the observer-target distance (OT) relative to the gun-target distance (GT), then multiplying that ratio by the number of mils of deflection needed. This method accounts for the additional distance the projectile has to travel, ensuring that the corrections are proportional based on the distance difference between the observer and the target concerning the gun's position.

Utilizing this formula allows the artillery crew to adjust the deflection accurately so that the rounds land on target, considering the longer flight time and the effects of factors like wind. This proportional adjustment is essential for precise target engagement, especially at longer ranges where the distance can significantly affect the projectile's path.

The other methods either do not incorporate both the observer and gun metrics correctly or fail to provide a precise adjustment for mils, leading to potential inaccuracies in hitting the target. By focusing on the ratio of these distances and applying it to the deflection measured in mils, option B provides a reliable means to calculate the necessary correction for effective fire support.