Gluten and protein contents

I'm using Progressive Baker bread flour by Horizon Milling.  I bought it in a 50# bag, and there is no information on the bag about protein or gluten content, except the list of nutritional info for 1 cup of flour.  It says one cup has 12 g of protein.  How do I figure it all out.   

I can get King Arthur Special - Unenriched bread flour from this same source.  Would that be a better over all choice?  I also have whole wheat and rye flours (50# each), so I don't want but one kind of other bulk flour.  The space thing is killing me.  Thanks for your advice.

Syb