In general the 5-day minimum begins when you consider yourself niddah. Based on your description it sounds like you only considered yourself niddah at 6:55 when you were sure of the flow. However, given the extenuating circumstances, in this case you may count the 5-day minimum from the earlier bleeding.
For the future, if you know your period is starting and you want to start the 5-day minimum already, you can perform a bedikah to definitely render yourself niddah. Alternatively, if you follow Ashkenazi custom, you could wipe immediately after urinating. You should only do this if you are certain your period is starting; otherwise, it could make you niddah unnecessarily.
Please feel free to get back to us with any further questions.
B’Hatzlacha!