Question about 5th night free with points

I’m looking to transfer some of my Amex points for a reward stay. My question is, do I need to transfer enough points in my balance for all five nights to even book the stay and get the 5th night free, or only enough points for 4?

For example, if the hotel I want is 20k Hilton points per night, do I need to transfer 100k worth of points to get the 5th night free and end up with 20k unused in my account, or only transfer 80k and it will let me book five nights? I assumed the latter, but someone told me otherwise and would like a definitive answer before I transfer the points. Thanks!!