I would trust your testing over the pool store, any day. The CYA test is very subjective and differences in lghting can have very large differences in your results. That is why it's important toduplicate the conditions during testing as close as possible. That being said, for every 10 ppm of chlorine you got from the trichor, you added 6ppm of CYA. So if you were gone for a couple of days, a 50ppm increase is doubtful. But if you were gone for a few weeks, it could happen....