Oh my....if you are "focusing on getting your horse's head set right," I'm sorry to tell you that you're not really doing much dressage. I know your question is about stirrup length, but if you are constantly fighting your position or your saddle, or just generally ignoring your position in favor of putting your horse's head in a certain spot, you are going to find it next to impossible to ever have the horse working correctly.
As for your original question, your leathers are probably too long. People see that nearly straight leg on the GP riders and think that's how everyone should be riding, but it takes so much strength, balance, and flexibility to be able to sit with one's leg draped that well. As you're working toward a more correct seat and leg, you are much better off, IMO, to bring your stirrups up a couple holes and quit reaching for them.