Quote from Chabah:
If you want to cut it very close I would recommend you have extended trading hours enabled. I believe this is an extra fee (been a while since I did it) but would be worth it.
Personally I use a "LastBar" on a 10 or 15 minute minute chart and don't try to be too cute with the close. Depending on your broker certain trades may not fill automatically - specifically shorts. So if you want to get a manual fill you'll want a couple minutes to call. Just closing positions should never require that, of course.
Good luck,
Damien