Airforce Talon 177 best shot string

Hi all,
I am starting to sight in my talon 177 and was wondering what power level would be best for around 20 foot pounds energy. I do not have a chronograph. I am sighting it in with 11.5 grain pellets but also have 16 grain pellets. Any advice would be helpful. I have it set at 5 power setting right now and it seems it drops from 3000 psi to 2000 with around twenty shots maybe less.
Any pointers would be great.