How long before Abraham did the flood occur?
The flood occurred about 400 years before Abraham in round figures. Abraham is placed at 2000 BC, halfway between Adam's creation at 4004 BC and Christ, and backing away from Abraham about 400 years brings us to the time of the flood.
Les explains that he divides the timeline in round terms so it is memorable: from 4000 BC to the time of Christ, halfway between is the appearance of Abram, or Abraham, at 2000 BC. Then, when you back away from Abraham about 400 years in round figures, that brings us back to the time of the flood. So the flood is placed roughly 400 years before Abraham.