I have done the math and the pattern board work and even killed some birds from time to time. It really depends on your hunting style and the game and the time of year:
Lead Pheasants over flusher - cheap 6s if you want to practice tracking, premium 5s most of the time, premium 4s for late season
Steel small ducks over dekes - 4s over 3s if you can get them close enough.
Steel medium ducks and calm pheasants- 3s over 2s
Steel jump shooting and late season pheasants- 1s over 2s, 1's will penetrate from the back, BB is too course