I wonder how much more ascorbic acid to add when trying 75% hydration with the 00 Americana flour.
That's a good question. The 00 Americana flour does not have any ascorbic acid. And it is not entirely clear whether the protein content recited in its specs at http://caputoflour.com/wp-content/uploads/2016/07/00-Americana-Flour-SPECS.pdf
is on a dry matter basis rather than on a 14% moisture basis as used as a standard in the U.S. I mention this latter point since the amount of ascorbic acid that millers add to their flours appears to bear a relationship to the protein content. For example, I looked at several General Mills flours that GM identified as having ascorbic acid in the GM pdf document at http://www.generalmillscf.com/~/media/Files/Industry-Resources/Pizzeria/exploring-products/flour-portfolio.ashx
. The amounts of ascorbic acid contained in those flours are as follows:
King Kaiser, protein 14.2%, 45-55 ppm ascorbic acid
Supreme, protein 13.6%, 45-55 ppm ascorbic acid
Superlative, protein 12.7%, 30-40 ppm ascorbic acid
Big Loaf, protein 12%, 20-30 ppm ascorbic acid
If the protein content of the 00 Americana flour is on a dry matter basis, the amount of ascorbic acid to use might be around 30-40 ppm, or 0.003%-0.004% when stated as a percent. Presumably that would be as a percent of the formula flour. Once you have your dough formulation in hand, you might try that range of percents to see how it converts to weight or volume. At this point, I do not see a need to use one of the dough calculation tools since the amount of ascorbic acid should be a very small part of the overall dough weight.