If everything is predestined, whatever you choose is your destiny. Which means you get to choose your destiny. Even if the decision is already determined, your decision process is a part of that and whatever you decide is what becomes the future (or present). Predetermination is irrelevant unless it can be seen beforehand, but if it could, that knowledge could be acted on to change it. So either you can see the future and change it, or you can't so there's no functional difference between it being indeterminate.