Negative pressure required in Vacuum leak test for blister packs

Please correct me if I am wrong but I am unable to understand the mandatory pressure value defined at the information given on a different page of this site.
[Ref.: SOP for Leak Test of Strip, Blister and Alu – Alu Blister Sealing : Pharmaceutical Guidelines]
In this SOP as defined by the author the vacuum test pressure is 15 inches of Hg. please guide me in which international standard or by which FDA rules/regulations this value is derived?. why cant we test at lower pressure or even for for example say 5inches of Hg? are there any internationally accepted supporting documents available for this specific value.
and in the comments some blogger has different opinion. Can some one please help me to understand in which standard document these values are defined?

Thank you in advance.