I remember discussing that same question with my classmates when I was in nursing school
! You are actually exactly right.
If the medication volume is LESS than 5ml, then you DO NOT add it to the total volume. If the medication volume is MORE than 5ml, then you DO add it to the total volume. This helps make sure that the pt is getting the full dose of their medication, at the proper dilution.
It's also a good idea to check with your nursing dept, just to see if they have their own guidelines for testing purposes. But that is the rule I have always used, and it is confirmed in the text that I use for a math reference ("Clinical Calculations", by Kee & Marshall, 5th edition, pg 212).