taking the pH of ro water makes no sense. When you mix cal-mag to your base nutes it should stabalize the pH. Make sure you are using the correct ratio of cal-mag to base nutes. You may still need to use ph up or down but unlikely if your ratios are correct. Start taking the ph after you mix your nutes. First cal-mag then base nutes and then check pH, If it is off you may need to adjust your ratios. If you are confident your ratio is correct, adjust pH after you mix the nutes.