Dear Rabbi Simon,
I was wondering why you are starting the fast of 17 Tamuz (this past Tuesday 11 July 2017) at 3.43 am, when most sources I can find say 1.35am?
The simple answer to your question is that I am following the straightforward approach of Dawn = 72 minutes before sunrise. Others calculate based on degrees below the horizon, which this time of year, at this latitude, makes a big difference.
When it comes to beginning a rabbinic fast such as 17 Tamuz, IMO one can rely on the generally accepted formula which fixes halakhic dawn at 72 minutes before sunrise, any time of year. So you can enjoy a snack until that time. When you go sleep the night before, you should have in mind that you are not beginning the fast until 3:43am.
Bon appetit and have a manageable and meaningful fast
Rabbi Rashi Simon