I believe that objects cannot be transitioned into the Glacier Storage class until 60 days after their upload date. So at the very least, you will have 60 days

I believe that objects cannot be transitioned into the Glacier Storage class until 60 days after their upload date. So at the very least, you will have 60 days of latency (and thus ~$20 of monthly fees) before you can extract your other data. Additionally, I wouldn't be surprised if the 5% is also based on a storage measurement that is pro-rated for the month. So I would let the 1200 GB of data sit in Glacier Storage for another month before extracting anything, just to be (more) safe.