Thus far, two of the recipes have called for high-gluten flour---the Light Rye Bread and the Bagels. Neither is a surprise. Since rye has less gluten, a high-gluten flour often added to "make up the difference", so to speak. Bagels need the added gluten to help provide the added chewiness.
At this point, I've only made the Hamelman bagels once. I'll add a post once I've had an opportunity to revisit them. I've made the rye twice and will post on it in the next day or so. With the bagels, I used up the last of the high-gluten flour I had. This is something I can't find locally and have yet ordered any more. So with the rye bread, I've had to "mix" my own high-gluten flour using bread flour and vital wheat gluten, both items readily available locally. This seems to have worked well, so I thought I'd describe the calculation I made. (I plan to use this technique when I revisit the bagels. I'll post my results when I do.)
For those of you who might not be familiar with it, Baking Science & Technology by the late E.J.Pyler is often cited as the definitive book on the science of bread. A couple of years ago Sosland Publishing enlisted L.A.Gorton to bring out a fourth edition of the book. This is a massive two volume work. I only have the first volume (772 pages) which set me back $165.
Pyler provides a table for raising the gluten level in flour through the addition of vital wheat gluten. The table has a row for starting protein levels from 6% to 16%. It has a column for target levels from 7% to 17%. Each entry in the table give the amount by weight of vital wheat gluten that needs to be added to raise the protein content to the target level. I used this table to determine how much vital wheat gluten to add. (I'm tacitly assuming that all vital wheat gluten is the same, or, at least, that the vital wheat gluten I have is typical.)
For my starting point, I'm using King Arthur Bread Flour with a protein content of 12.7%. For my target, I'm using King Arthur High-gluten Flour which has a protein content of 14.2%, or a net increase of 1.5%. The nearest table entry in Pyler says to add 1.49 to increase from 13% to 14%. So, extrapolating, an addition of 1.5 x 1.49 = 2.235 is needed. (Pyler uses pounds but I see no reason why units matter as long as measurements are done consistently.) In other words, to go from bread flour to high-gluten flour, add 2.235 grams of vital wheat gluten to ever 100 grams of bread flour you are using.
Extrapolating a bit further, since 100 of every 102.235 grams of the mix is bread flour, you'll multiple the amount of high-gluten flour called for by 0.978 to determine the amount of bread flour to use. Since 2.235 grams of every 102.235 grams of the mix is vital wheat gluten, multiple the amount of high-gluten flour called for by 0.0219 to determine the amount of vital wheat gluten to use.
For example, the Light Rye Bread calls for 1 lb, 11.2 oz or 772 grams of high-gluten flour. I would use 772 x 0.978 = 755 grams of bread flour and 772 x 0.0219 = 17 grams of vital wheat gluten. These are the weights I used when making the rye bread and they seemed to work fine.