Just a thought regarding your spare magazine.
I would recommend AGAINST carrying your spare with the gun, and here is why.
You use one hand to draw the weapon, if you want to get the spare mag, you have to switch the gun to the other hand, then draw the mag. Most people would then switch back to their primary hand for handling the gun. In a fight, this could be too much time lost.
I carry my spare in a way that my alternate hand can draw the spare as I am drawing my sidearm with my primary. IMO, it puts me in a better position if it gets really bad, really quick.