This thread has drifted a little bit so I would like to get back to CH testing.
Cayman,
Time after time, year after year, post after post, the CH testing method used by the Taylor K-2006 and the TF-100 have proven to be the MOST accurate method you can get. You get 425 (more than once) and your PB did the test and got 425. That is within an acceptable range and I would be willing to bet that is your CH.
The pool stores testing has conversely proven over the years to be completely unreliable so you should doubt them LONG before you doubt your own testing. Their is nothing in your other test results that suggests you are making testing errors so believe in yourself.
Don't overthink some of the minor variations. CH has a range of 350 - 450 (and there is a lot of cushion built in......many manage pools with CH 700+) so because you are using a softener on your refill water, your CH is well under control. Trust your own testing, stay within TFP ranges and enjoy your pool!