For tacheometer: The stadia method is based on the principle that the ratio of the perpendicular to the base is constant in a similar isosceles triangle. 
As per the principle of stadia method, the horizontal distance method is given by: 
D = KS + C 
Where, multiplying constant(k) = f/i 
Additive constant (C) = (f + d) 
s = Staff intercept. 
If a tachometer is fitted with an anallatic lens, its additive constant is zero and the multiplicative constant is 100