Generally I make two complete runs up and down the outside of a bar before i make a selection of where is my best choice to cross. Once you are committed there is no turning back so make that choice twice. It never amazes me how much a bar's conditions can change for a particular crossing location on 5 minutes.
Several times i have made a mental note of where to cross on the first passing outside only to see it turn to crap on the next passing.
Your window of opportunity for the best passage has always passed when you have seen the perfect entry wave. After that you make do with what your selection delivers you and do your best to do it safely.
Jack.