How to convert between flours? As we've seen in previous questions, certain flours have different gluten contents. Time and time again, I find reviews of King Arthur Flour recipes indicating that I really must use their flour if I want the recipe to come out right, as any other flour will make it come out horribly wrong. Is there a way to convert a bread recipe to use a more widely available brand, given that my supermarket actually sells gluten in a bag?

ETA: Also, is there a way to find out what makes a given brand different so I can evaluate it?
If you are simply looking to convert gluten contents then you can certainly always go higher by adding the correct proportion of vital wheat gluten to dough to get the percentage of gluten that you want.

However there are many other differences between flour bags in terms of processing.  How is the flour ground?  What wheat variety is it from?  Is it white or whole?  There are a lot of variables.

I do speak as someone who has found that Gold Medal is perfectly acceptable for all my baking needs and also routinely uses store brand flour.  Personally I suggest simply trying whatever flour you want to try (not including substituting  bread for cake or something along those lines, that definitely will cause issues) and seeing how the recipe goes.  Determine if a higher-end flour brand is for you yourself.