I have not used the sure grow cubes so. I would make sure the cube is moist but not soaking wet. If the cube is moist the root seeks water and if too wet the root dies from pythium. In the begining I have to water rockwool, root riots and rapid rooters by hand and only a few drops just to barely keep them moist.

Your watering time is right but not untill you have roots coming out if the cube. Then water away.