throw it throw it throw it. It will click. Throwing a good change up is a feeling. As soon as you release the ball you'll know.
I've always told guys to throw from 90 feet, 80 feet, 70 feet. just throw the hell out of it until you feel like you have it mastered. maybe some days only throw the ball with the change up grip. all of these tactics I've used and they've all helped me get the consistent feel for the pitch