I generally use the same envelope between buys and sells. As for pre market trades, I screen for news and avoid it, as long as there is no news then ecn's don't matter IMO.
As for optimal exit....that will come with time. If you are going to try and maximize your gains every day, you have better be prepared to run the strategy for two weeks in a row and lose. Not many can do this, for now, I would focus soley on making money, taking the scalps, making a few hundred and building your account, confidence, and really learning how the strategy performs under different market conditions. Once you achieve a level of mastery(backed by profits), then I would start to work on maximizing profits.
Good luck!