I have updated the tags for better visibility . I will move this to software for now, can you let us know the revision of Openlab you are working with?
Open Lab CDS version C.01.03 (37)
I cannot weigh in on your methodology, but here is a link to the Reference to Operation Principles guide for OpenLab CDS ChemStation: https://www.agilent.com/cs/library/usermanuals/public/CDS_CS-references.pdf
The section outlining the math of the ISTD calculations begins on page 92. Please let us know if you have any questions.