But to answer the question. 8" is enought, but just because you have 8" of blood does not mean 8" of penetration. Problem with the shoulder is it doesn't bleed. We have all been there. It sucks. Best thing you could have done is wait at least 3 hours. Espeically with marginal hit. That way he would have laid down for the dirt nap.
But again, not bagging you. Sorry to hear about your deer.