Wow.. I haven't added 14 oz since after the startup in my gunite pool! (The startup guy added a full gallon which was too much, but came up fast with the brushing we had to do.) I would seriously consider an electronic pH meter. I will step on some toes here but the Taylor pH test sucks, the electronic meter is the best but even another brand of titration test seems to work better. I kinda feel the same way about the CYA test in the kit too as I literally have strips that seem to work just as well. The Taylor pH works but it's about +/-0.15 accuracy which I don't think is great on something that is a logarithmic scale. The other tests are superior except for TA where the competition works differently but with exactly the same results.
In a fiberglass tub you really are only concerned about FC/CC and pH/TA. I am really concerned about your adding that much muriatic acid. When I dose 31% muriatic, I do it a teaspoon at a time for my 245 gallon tub. Literally 2 tsps (0.3 fluid ounces more or less) will drop pH by 0.2 in that tub... so in yours it would take a tablespoon to do that, since it is twice as large (or about 0.6 fl oz). I actually take a 64 oz gas station drink cup, fill it up with RO water and put in a teaspoon at a time to do adjustments, so I don't risk marring anything... I also cut my liquid chlorine the same way in the tub to avoid bleach spots.
So, in theory anyway, in a 425 gallon tub, 14 fl oz will lower TA by 129 and pH by 7.9. I don't think your fill water could be that hard, even in Texas.
Go and buy an electronic pH meter, DISTILLED water and packs of the buffered standards for the meter. Most buffered standards are now 6.86 at 25C and 4.00 at 25C. Adjust your meter initially with both standards, and then any time you take a reading (once a day if needed) adjust the meter to the 6.86 pH standard. If you do this your pH accuracy will be about +/-0.05 on that meter. But you really should calibrate every time you use it.
DO NOT ADD CHLORINE FIRST. It will show a false very high pH reading for at least an hour after adding chlorine in a tub "that small" (compared to a pool). Do all of your water testing before adding any chemicals. You can add acid and recheck after maybe 10 minutes with the pump circulating in a tub, but never do this with chlorine.
I think you are actually looking at a very acidic hot tub now, and something has been wrong with your testing. Be prepared to refill.
