Good advice, with regards to measuring the specific apples you're using yourself, as levels of sugars, TA, tannins, pH, etc. will vary depending on the terroir where they were grown, as well as seasonal weather. For example, after a prolonged period of freeze and near-freeze events surrounding bloom and fruitset last year in our orchard, we were amazed to find that some of our varieties produced good quantities of fruit at all, but that they also had strangely high levels of tannins. We contacted a university researcher to try and understand this and he explained that our trees had probably responded to the unusual cold stress by pumping up tannin production. For the tannin levels to shoot up that perceptibly high (some of the dessert apples were too bitter to eat) is pretty unusual, so I'm not suggesting that this is something you'll run into often, but just as a good example of how localized events can change the chemistry of your fruit in ways that you can't predict. If you're buying apples from someone, not growing them yourself, get to know the growers and talk to them about how their season went, and if they have values available for the fruit they're selling. Many cider apple orchardists track at least some of the values you're interested in, in order to determine peak ripeness.
That said, if we're evaluating for ripeness on a particular variety and want an idea of whether we're in the ballpark of Brix normalcy for that variety, we sometimes refer to compilations of variety-specific values done by Walden Heights (Vermont)
https://waldenheightsnursery.com/research-articles/sare-1/ or Washington State University's database of values for cider apple varieties grown in that state (
https://cider.wsu.edu/ciderweb/).
If you see variance from what you're looking at with the fruit you have, just keep in mind where the apples were grown and what sort of growing season they had. Our orchard is in the hot North Carolina Piedmont, and it's not uncommon to see higher Brix values on varieties grown here as opposed to, say a cooler climate in New England.
