I did a bit of research on this since I couldn't get a solid answer from the local filling stations that I spoke with.
I will post here what I believe is the correct answer to this question in order to assist someone who has a similar question.
Blue Rhino has a label that claims the contents of the tank are 15lbs. It also says that you should not fill a tank more than 80% of its capacity. I initially thought to myself a 20 LB tank @ 80% = 16LB and figured that BR fills them with one pound less to save a buck. After reading dozens upon dozens of posts, I kept on finding that almost everyone was thinking along the same lines.
This changed when I learned about the water capacity of a tank. Every tank has a WC and Tare weight stamped on the handle of the tank. The Water capacity is how much water a tank is capable of holding at 100% capacity.
The WC stamped on my tank read 47.6lbs. This means that if you were to fill this tank completely with water, it would weigh 47.6 lbs.
Knowing the weigh of water is 8.35 lbs per gallon, 47.6 / 8.35 = 5.7 gallons
Simply multiply 5.7 gallons x the 80% capacity and you get 4.56 gallons.
or to put it another way:
Liquid propane weight approx 4.27 lbs per gallon (depending on temperature)
4.6 gallons * 4.27 lb per gallon = 19.64 lbs
So to answer my question, with the Overfill Protection Device installed, I should be able to fill my tank with just under 20lbs of propane, not 16lbs as many many people believe.
Anyone verify this? Can add to it or take away?