I go by a 40C/30P/30F ratio. It was mentioned, and should be mentioned again that these ratios have nothing to due with fat loss. As long as you are burning more calories then you consume then you will lose fat.
I use this ratio because I am currently eating at a 1000 Calorie a day deficit. As such I comsume a lot of protien so that I minimize my muscle loss while I am cutting. The mix of carbs and protein help me to control my appetite and diet. When I go below on my Carbs, I crave them like crazy and when I go below the fat percentage I am constantly hungry.
Using this ratio in light of my calorie intake has helped me to comfortably lose 47lbs since November. Everyone is different though so there is no real right or wrong ratio to use.