I have three related questions about doing the dumbbell pull overs across the bench.
How low should I keep my hips when doing the exercise?
How should I breath when I do the exercise?
Should I retract my scapula when doing this exercise?
There's no concrete answer but you want your hips to be at a height so that your upper back lies flat and stable on the bench. That will mean having your hips at a height somehwere between the floor and the height of the bench.
The valsalva maneuver. That's the technical term for taking a deep breath at the beginning of the rep and then exhaling during the concentric. This breathing method is used by most people for most heavy lifts.
And yes. Bench pullovers are primarily for the chest and so retracted scapula and a tight back are proper form, as for all chest exercises.