I would try 0.75 units.
However, I would switch to a better insulin like Lantus/Levemir or the pet insulin ProZinc. The response you are seeing is typical of N. It really does not last 12 hours and usually peaks at about 5 hours. With N you have a relatively high PS like you are seeing when you get a good (low) minimum. With Lantus/Levemir and ProZinc you generally have a flatter curve.