Fasting starts from 'fajr', and ends at 'maghreb'. This means that you are able to eat, drink, etc, until just before fajr time. The issue whether you are able to accurately and precisely say when fajr time starts.
This is why the best thing to do is to stop a few minutes prior to fajr time, refraining from any of the invalidators of fasting. By doing this, you will be sure that you have not done anything that invalidates your fast.
As for how many minutes this should be, that is up to you, as long as you are trying to be as accurate as possible, and observing as much precaution as possible.
And Allah knows best.