My rule of thumb is 2 hours when going to work, 1 hour going home.

I will take a shorter connection on the way home if it is the only/best thing available.

I will take a shorter connection on the way if there is sufficient protection for the destination.

I know ATL like the back of my hand and would never book 40. Have I made it in less than 40? Many, many times. But not by choice.

Unless the flight is early (which happens frequently) there is not room for error.

I almost had the spend the night in ATL last week for the first time in years. Why? Because of the way I book. I have co-workers who miss-connect and lose their bags and bellyache about it all the time......because they don't know how to book.