You might find this article interesting: www.sparkpeople.com/resource/nutrition_art
It outlines the ranges of carbs / fats / protein that Spark promotes as being healthy. The "ideal" shown on the Daily Feedback chart as 50/30/20 is just around the centre of each range, but there is still a lot of room for adjustments around them.
I wouldn't get your knickers in a twist about hitting exactly the 50/30/20 - that's just an easy guideline to give you an idea of about where to be.
Hope this helps!