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.