Better a little too much protein than not enough.
I have been really obsessed with protein amounts for a long time so don't mind me if this seems too elaborate.
You need about 30-40 g(safe side) glucose per day and it can come from either protein or carbs. If it's carbs, count grams. If it's protein, take off 30% that goes to digesting the protein (carbs takes 4 or so % I believe so it's negligent), that way you need to eat that much more protein than carbs IF you are doing low carb. You don't want to not get enough bodily protein because it is being used as glucose for the brain.
Then calculate the amount of protein your body needs for repairs and such, again take off 30% to digesting it (better safe than sorry?). Add this to the previous number, and try to go a little higher just to be safe. Fill the rest with fat. If you want to fill it with some carbs, your protein requirements will go down as per part 1 of this formula, of course there is a minimum and you want to stay well above that :-)