 The target range at the time of projectile impact can be estimated using Equation 1, which is illustrated in Figure 3.
where 

The exact prediction of the target range at the time of projectile impact is difficult because it requires knowing the projectile time of flight, which is a function of the projected target position.While this calculation can be performed using a trial and error approach, this was not a practical approach with the analog computer hardware available during World War II.In the case of the Ford Rangekeeper Mk 1, the time of flight was approximated by assuming the time of flight was linearly proportional to range, as is shown in Equation 2. 

where 

The assumption of TOF being linearly proportional to range is a crude one and could be improved through the use of more sophisticated means of function evaluation.