That's the equation from USP chapter 621 which is supposed to yield the permitted %RSD for a series of injections a part of system suitability.
The full text is:
"For the Assay in a drug substance monograph, where the value is 100% for the pure substance, and no maximum relative standard deviation is stated, the maximum permitted %RSD is calculated for a series of injections of the reference solution:
%RSD = KB x sqr(n) / t(90%, n-1)
where K is a constant (0.349), obtained from the expression K = (0.6/ sqr(2) x t (90%, 5/sqr(6)), in which 0.6/ sqr(2) represents the required percentage relative standard deviation after six injections for B = 1.0; B is the upper limit given in the definition of the individual monograph minus 100%; n is the number of replicate injections of the reference solution; and t (90%, n-1) is the Student’s t at the 90% probability level (double sided) with n-1 degrees of freedom."
There obviously is an explanation for K but my statistical knowledge is too limited to interpret it
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