If you're really concerned about your crown, use twice daily applications of minoxidil. With a NW4 crown you shouldn't be too worried about it messing up your hair with greasiness etc. Better yet, use the foam. I think with the success you're seeing on your crown, a little more rogaine on the crown might work. One application a day probably isn't doing much. It'd probably be even better, if you want to maintain one mL a day, too apply half a mL twice daily. Ensures a constant flow of minoxidil.
You sound particularly concerned about the crown. Maybe try a big gun like topical spirolactone?