Get the list of all pre-requisites, see the grades you got in said pre-reqs, throw it in a GPA calculator, and it'll give you the GPA. You're probably doing it right 🙂
As for the last 60 hours, get the last sixty credits you took and throw it in a GPA calculator and voila! But little tip - if your last 60 GPA doesn't add up to 60 credits exactly (aka - it ends up being like 62), they'll count that WHOLE semester in your GPA (at least that was what the lady from EKU told me.