How to set the standard solution in a related subst method?

[AWMFC]

I would like to know which is the best approach in choosing the concentration of the standard solution for a HPLC related substances method to use in routine. Normally after performing linearity, range and accuracy during validation, in routine a single point calibration is chosen (if intercept in linearity resulted negligible) or a 3 points calibration. We usually set the concentration of the single point calibration solution at the specification limit of the impurity (or, in the same way, we fix the central point of the 3 points calibration solutions at the specification limit of the impurity). I know other companies that set the concentration of the standard solution at a value < o > of the specification limit. Which is the correct approach?

[HPLC chemist]

Set the concentration of the RS standard solution at your specification limit. The standard deviation/error will be about the same as an OOS sample that is borderline.