A couple of suggestions to ensure your testing is as accurate and repeatable as possible.
- If you don't already have one, definitely purchase a SpeedStir, magnetic stirrer. The SpeedStir is universally acclaimed by members of this forum, and that's for a very good reason. After working with a SpeedStir for a while, you quickly realize how poor hand agitation can be, especially for TA and CH tests. You really need good mixing on those tests to "force" the color changes to happen within a normal drop pace. With mechanical agitation I've found it's way too easy to add extra drops because insufficient agitation slows the color change.
- As I mentioned above, the SpeedStir makes sure the mixing is at a pace that keeps up with your drop additions. Especially for the CH test, make sure you slow down your drops a little as you approach what you believe will be your final number. When you have high CH, there's a tendency to speed up your drop pace because there's so many drops to add. The alernate 10cc test is a good way of saving on reagents and quickens the overall test time.
Cynical thought. If I were a pool builder who knew the variability and inaccuracies of pool store testing, it would be to my potential benifit to have the customer send me reports that might document poor pool care, even if in reality, the care was perfect; because when push comes to shove, those documents could be used against the pool owner. On the other hand, as others have suggested, it wouldn't be too hard to prove inaccurate pool store testing by comparison sampling at multiple pool stores. If it was me, and because the PB has unilaterally imposed pool store testing, I'd perform and present him with the multiple store comparison sample testing now, rather than later.