I dropped mine by 0.1V
This meant I was putting 82% back in to my batteries after a flight. It increased my flight time by a small amount but it varies so much I've not bothered actually timing it.
There are many factors that you have to consider, such as total flying weight, temperature, humidity, altitude and most importantly how you fly your Phantom BUT the main concern is that you should set the values in such a way that after the warning and landing your battery pack should have 20% or more left. This way the pack will have a long service life for you which is much better than get an extra minute out of the flight time. In fact, I set it for less flight time but more extra time after the warning so that I have plenty of time to bring my Phantom back, especially when I fly over water!
I use the gadget above to check the pack capacity.