I've been using the TFP PoolMath on this sight. Yesterday I stumbled upon poolcalculator.com, which appears to be the same calculator. Except with the same raw numbers plugged in, PoolMath is telling me my shock level is 31, while poolcalculator.com calculates 21. Pretty big difference for what seems to be the same calculator. Anyone else noticed this? Which is correct? I'm doing a SLAM and want to make sure I'm at the proper FC level.