Question on how to bet when the flop misses

I have trouble knowing how much to bet to try to steal a pot in these circumstances:
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I have position against 1-2 other players.
I bet before the pot, a standard bet of 3-4x bb.
The flop misses me, but it is checked to me and I think the flop missed the other players.[/list]
Example:

Preflop: MP calls BB, I raise to 4xBB from LP, SB folds, BB calls, MP calls.
Pot = 12.5 BB.
Post flop: BB checks, MP Checks, I raise... ?

I would like to automatically bet without thinking about the amount. I want to be able to take down the pot, but not over commit myself in case I'm called or check raised.