You should be able to perform 5 reps per side with 50% of your bodyweight. That would be a 100-pound dumbbell for a 200 pound man.
Before you throw a tantrum, yes, the barbell bench is absolutely valuable. The problem is, you can get away with pushing heavy weights even if you have glaring weaknesses and faulty mechanics.
When you become "efficient" despite these issues, it's only a matter of time until your strength reaches a glass ceiling and you get hurt. Once you refine your pressing mechanics and eliminate or decrease imbalances, you'll stay healthy and build more strength over the long haul.
The one-arm dumbbell bench press exposes any weak links and gives you immediate feedback on strength discrepancies between sides. With this info, you're able to train each side individually and bring up weak points in your shoulders, pecs, and triceps. As a bonus, the unilateral load forces your core to work overtime to avoid the humiliation of falling off the bench.
Take a brief hiatus from the barbell, dust off your dumbbells, and get re-acquainted with them. Perform the exercise with one arm or alternate sides on each rep, focusing on slowing down the eccentric (negative) portion of the movement and mixing in iso-holds at various positions.