This well known rule is usually given as:

The base must not exceed  1/50  of the distance to the subject.

B < D/50

     This rule has been calculated and set in 1923 by E. Colardeau who stated:

" The choosen base must be the fiftyth part of the distance to which stands the first plans of the landscape "

Replacing  Bmax = D/50 in the SR formula, we obtain:

SRmax = 1 / 50.k

With k = 0.0004 this is     SRmax = 50

It is very interesting to see that my own observations based on the Stereoscopic ratio, stating that SR>50 is excessive stereoscopy, is the same than the 1/50 rule.

But the 1/50 rule doesn't determine which base is best to set. The SR formula gives it, by calculating:

B =  SR . k . D . (D+L) / L