Half sector (half deflection) is at 0.12 times the GP angle. For a three degree GP, this means 0.36 degrees. This gives 0.36/2.5 degrees/dot, or 0.144 degrees/dot.
One dot below gives a 2.88 degree glide path.
The difference x between on-GP height and one dot below height, h_1b, is thus, with d being the distance from the touch down point,
x = d*tan(3 deg) - h_1b (1)
The distance is given by h_1b as
d = h_1b/tan(2.88 deg) (2)
Insert (2) into (1) and you get
x = h_1b/tan(2.88 deg)*tan(3 deg) - h_1b/tan(2.88 deg)*tan(2.88 deg) =
= h_1b*(tan(3 deg)/tan(2.88 deg) - 1)
Or: The height below nominal GP when at one dot below is given by the actual height times 0.0418.
x = h_1b * 0.418.
100 feet -> 100*0.418 = 4.17 feet difference.
200 feet -> 200*0.418 = 8.34 feet difference.
300 feet -> 300*0.418 = 12.5 feet difference.
Using d*tan(0.12 degrees) instead of d*(tan(3 degrees) - tan(2.88 deg)) introduces an error which has an impact if you are going for three significant digits.