As an amendment to your potting soil, the recommended rate is: 9 grams/1 tbsp/15ml per 1 gallon of soil/4 liters. If you would like to mix larger quantities (usually in a toy swimming pool or on a sheet of plastic) mix in 4tbs (60ml) per cubic foot, so approximately 16-20 tbs (240-300ml) per bale of Sunshine or Pro-Mix. Always wear a mask or respirator, gloves, and goggles (especially the mask or respirator) as directed. There isn’t much information out there on Calcium peroxide being used in recirculating hydroponic systems. I would think that small amounts in rockwool cubes and slabs might be O.K. Rockwool tends to be slightly higher in pH than required. This can be desirable when adding high quantities of phosphorous during later stages of flowering. These are often fertilizers that reduce pH. Further pH adjustment is also used in the form of “pH down” to maintain a level of pH5.5 in the reservoir, pH in the slab should be slightly higher.