I recently bought this tester Amazon.com: Oakton Eco Tester TDS High: Patio, Lawn & Garden the oakton ph2. Its a three point tester and I forgot to get the 10.0 calibration soulution. Cant hit the store right now but am going to calibrate the two points. I'll get some of the other solution in a few days. I am just wondering how accurate you think it willl be with only a 4 and a 7 calibration if at all.