Thanks for your response, waterbear. Our hot tub is acrylic, about 3 years old; we use bromine floaters instead of chlorine. Yes, we are using strips...just bought some new AquaCheck strips, in fact from the spa place (different brand, we had been using HTH) - we will get the Taylor strips you recommend. The spa is about 300 gallons. Using our test strips we got the following readings, done 3X to make sure (including once on our well water, which we use to fill):
Total hardness: zero (I assume this is TC, correct?)
Total bromine: zero (because we haven't put any in yet; our last water test of the tub water before filling it was 'very high' (>20)
Total alkalinity: high (240)
pH: high (> 8.4)

When the spa guys tested our old tubwater, he said it was basically battery acid because of the high bromine content - I wasn't getting why we would get such a high bromine content yet the pH is also way high, which to me indicates it's highly basic, not acidic. Your explanation of the high alkalinity leading to high pH makes sense. And you are the second person to say the very low calcium is nothing to worry about in a hot tub. So I guess we just need to lower the alkalinity at this point, which should lower the pH, and then add in however much bromine we need to bring it up to the ideal level of ~5 or so. I will consult the forum for how best to lower the TA. Thanks again for your quick response!