I am super confused among Clearblue Digital OPK or WondFO! As per my research, ClearBlue Digital's refills aren't available without the device, that makes it very expensive ! What do you all do? I really can't afford to waste my time and money with ineffective OPKs as I am already 38. Looking for your valuable suggestions for an effective OPK , if its Clear Blue Digital, how do you buy refills?