Flank steaks are all roughly the same thickness and that is normally what you go by when setting time. For flank you are cooking well beyond the time needed to just cook the meat, you are extending the time to make it tender.
Kuldar, most cooks follow a new recipe all the way the first time they use it.
Of course you can adjust the recipe if your personal preference for that cut of meat is significantly different. Just have a good reason for making recipe changes or you will never know how good that recipe could have been.