Mixing Flour
Hey guys, first post here. I don't know if this is common practice, or not. But, I thought I would post this in case someone found it useful.
If you have two flours with differing gluten content, you can use this formula to find the amount of flour to replace to reach the desired gluten content as long as it is between the two original amounts.
( ( Larger - Target ) / ( ( 1 - Smaller / Larger) * Larger ) ) * Amount
Larger refers to the percent of gluten in the the flour with more gluten.
Smaller is the same only for the one with less gluten.
Target is the percent of gluten you wish to reach. It must be a number between larger and smaller.
And finally, amount is the weight/volume of flour you are using for your recipe.
The result will give you the amount of flour you need to replace. So, if you are using 200g of flour and the result of the formula was 50, you would use 50g of the less glutenous flour and 150g of the more glutenous.
Your question has a premise that gluten is the same in all flour types, but only varies by percentage. I'm not qualified to teach a course in cereal chemistry, but the reason gluten differs in more than quantity is that it consists of two proteins. These are glutenin and gliadin. When flour absorbs water and is mixed and kneaded, the two proteins form chains by interlocking. The result of those chains forming is what we call gluten.
But both proteins are not always present in a 50/50 mix. Furthermore, one is responsible for elasticity and the other for extensibility. Whichever protein dominates determines the dominant property. Spelt is a type of flour that is more extensible than wheat. If you were to mix it with another extensible type of flour you may find yourself coming up short on elasticity such that your dough may not hold a shape.
Suffice it to say, the amount of gluten AND the properties it exhibits will vary according to your mix of flour types.
Perhaps there are others here with far more knowledge, but I will simply wave the caution flag to say that there is a lot more to this than a percentage to be derived with a formula.
As a simple case, adding bread flour to all purpose flour or pastry flour will increase the gluten percentage, but beware of oversimplifying gluten.