One of the things that has long perplexed me is why King Arthur does not publish the absorption rates of their all-purpose and bread flours sold at the retail level. For those who are interested in this sort of thing, the absorption rate of a flour is a technical characteristic most closely related to the hydration rate. In the past, I did find materials at the professional section of the KA website that gave the absorption rates for several of their flours, but not for the all-purpose and bread flours that we so frequently use. A good example is this document: http://www.kingarthurflour.com/professional/specifications-conventional-bakery-flour.html.
Another example is this one, for its organic flours: http://www.kingarthurflour.com/professional/specifications-organic-bakery-flour.html.
Since I was unable to find the corresponding numbers for the KA all-purpose and bread flours after scouring the KA website and doing several online searches, I decided to call KA. Through voicemail exchanges with Tod Bramble, the flour guru at KA, I learned that the KA retail all-purpose flour has an absorption rate of 61% +/- 2%, and that the KA retail bread flour has an absorption rate of 62% +/- 2%. Many of our members who use the KA Sir Lancelot flour are already aware of its 63% +/- 2% absorption rate. I also learned that there are professional counterparts to the KA all-purpose and bread flours. The professional counterpart to the all-purpose flour is called Sir Galahad. For the bread flour, it is called Special.
My practice has been to use the rated absorption rates when in doubt. It is a conservative, but safe, approach. For example, if I were to suggest that one use 66% hydration for the Sir Lancelot flour (KASL), I know from experience and feedback that there will be many who will have a hard time handling the dough because it is too wet. So, 63% seems safe, although there will be those who will find even that value too high. I might add that there is an interesting correlation between the rated absorption rate and what is called the “operational absorption” rate, which is essentially the optimum hydration that produces the best results. I first learned of this correlation from member scpizza who was kind enough to send me a link to this article: http://www.bsimagazine.com/feature_stories_print.asp?ArticleID=37104.
As noted in the article, the operational absorption rate can be 2-4% higher than the rated absorption rate. That would suggest that a flour like the KASL can tolerate a sizable increase in hydration above its rated absorption of 63% +/- 2%. That might also help explain why some members are able to get, say, 65-70% hydration, with a high gluten flour like the KASL.
EDIT (1/16/14): To read the article at the above inoperative bsimagazine.com link, see Reply 9 at http://www.pizzamaking.com/forum/index.php/topic,8419.msg72940/topicseen.html#msg72940