How much coffee should I be using?

I have a 6 cup chemex and I use ground coffee from the grocery store (8oclock, Starbucks, etc.. I don’t own a coffee grinder) and if I follow the chemex instructions it says to use 1 tbsp per cup so if I use 6tbsp for a whole pot of coffee it turns out very dark and strong. Is this because I use store bought pre ground coffee or should I be using a different amount of coffee? Sorry for being a dumbass.