I would be REALLY REALLY careful not to hurt yourself with that thing. The concept of building "hand muscles" is painfully ridiculous. First off, you're strengthening tendons not muscles. Second, there should be a lesson learned from the famous pianist who tried to strengthen his fingers in order to be able to be the best pianist ever. Sadly he screwed up his fingers by keeping them pulled apart or something while he slept... bringing his career to shambles

Lesson:your fingers are fine, let nature run it's course and don't try to speed it up or you'll screw yourself over in the long run