Basically of the opposite of this question.
I'm writing during the 2020 pandemic, and bread flour is sold out everywhere because everyone (including me) is entertaining themselves at home by making bread. I did manage to find "high-gluten flour", which according to Epicurious seems to be more or less the same as bread flour but with a higher protein content:
Unbleached all-purpose has the lowest amount of protein, usually around 10.5%. Bread flour contains about 12 to 12.7%. High-gluten and whole-wheat flours have about 14% protein.
My recipe calls for white bread flour. (I'm making sourdough at 80% hydration, in case it matters.) Can I get there by mixing this high-gluten flour with AP flour? What proportions should I use? Any pitfalls I should be aware of?