This will be seem obvious to many of you, but if you use imperial measurements instead of the metric system , to find out how far the 25x distance for shooting the target, rather than do the 25 * FL (in mm) and then converting the result to inches, you'll get close enough by remembering that 25.4mm = 1 inch so use the focal length as your distance in inches.
In other words, instead of calculating 50mm x 25 = 1,250mm and then dividing 1,250/25.4 to get 49.213 (I am rounding) inches, use 50 inches as your sensor plane to target distance