Create my own high-gluten flour by mixing vital wheat gluten and bread/AP flour? Has anyone had success with giving their bread or all-purpose flour an extra gluten kick by adding vital wheat gluten?  I would like to know what ratio to mix the the two ingredients with, or perhaps some hints at a formula that can help me accomplish this, assuming it is feasible (i.e. mixing will produce a close substitute for high-gluten flour).  I'm also interested in any quality differences with this mixed approach vs. just using high-gluten flour in recipes.

You can assume I'm using King Arthur bread flour and King Arthur vital wheat gluten (or analogously King Arthur AP flour and vital wheat gluten), and that I'll be baking bread with the flours.

Note: this related question (When adding vital wheat gluten to a bread recipe, should one reduce the amount of flour equal to it?) mentions mixing, but doesn't go into details on ratios, etc.
Yes, and it is very easy. I do it all the time. 

You only need a very simple calculation. You don't even have to be precise. 

If you do want precision, you will have to find out 1) how much of your flour protein is gluten, 2) how much of your "vital wheat gluten" is gluten, and 3) how much gluten content you need for your recipe. Then use a simple rule-of-three calculation to get the amount needed to add. 

I usually don't bother, because 1) and 2) is information which is very hard to find, and due to the large difference in gluten weight and complete weight, imprecision from not using true proportions is minimal. What I do is: 


Look up the protein content of your flour (usually printed on the package), for example 9.6 grams per 100 grams 
Look up the gluten content needed for your bread recipe. If it is not specified, 12.5% is usual for bread flour. 
Add the difference in vital wheat gluten. In the example above, add 2.9 g of vital wheat gluten per 100 g of flour. 


This doesn't produce exactly 12.5% gluten content, but I think that it is within the tolerance of most recipes; indeed, not all commercial flours are exactly 12.5%, they vary with brand and season. 

I add the powder to the flour and mix it well before making the bread. If I am using a preferment, I add all the gluten to the preferment and make the non-fermenting part with AP flour only, so my gluten can benefit from longer autolysis. 

I have no direct comparison with "true" bread flour, as I have never used it. But my breads requiring bread flour turn out good for my standards. There is no problem with bad distribution, the dough turns out very smooth and evenly elastic. There is a pronounced difference to using AP flour only.