How do you choose a macronutrient split?

I have figured out how many calories I need for a healthy, slow fat loss, and I want to count my macro nutrients. But what is a good balance? Why chose a higher protein v. a higher carb ratio, etc. Right now, I am using zone at 40, 40, 20. Getting enough protein is a challenge, but I am doing it. Today I noticed a real lack of energy and wonder if this ratio is right for me...how do you go about picking a ratio? What are you basing it on?