Is the TF-100 CH test accurate when the CH is at a low level? The instruction says, I think, that when CH is low, divide the number in half - is this right? I thought that my water was soft - I've tested it with 2 local Leslie's and the results were both on the low side, like 180, but the TF-100 is twice of that. Is 1/2 of TF-100's number right?