I'm thinking you're tie-ing your offering directly to 20 mono (your main line w/NO leader at all)...right?
If yes,...I would use a 40# or 50# leader (Ande or Trilene MONO), swivel on one end, break-away clip on the other. I don't normally use anything longer than 3 feet. When wading with longer leaders...I can't dip the rod low enough to load my cast without my offering sitting in the water. I sometimes have to shorten the leader if the tide is rising and I don't want to back up...and the reason I stick to an average of 3 feet is because I don't want a diversity of leaders to choose from in the heat of the moment. I'll adjust as needed, if needed.
If no, disregard the above.
God Bless you too.