Post flop you should try to think in terms of bets relative to the size of the pot. This lets you easily control the odds you are giving to your opponent.

BB=100

Pot postflop contains 400

If you bet 200 "half pot" then you are giving your opponent 3:1 odds (400+200=600 compared to the 200 he has to call).

If you bet 400 "pot sized" then you are giving your opponent 2:1 odds (400+400=800 compared to the 400 he has to call).