Help understanding percentages from calories

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I'm new to cycling and to nutrition specifics in general. I'm 20 and have played soccer, and have been, fit all my life but a knee injury quickly stopped that, I can't play for an hour without dislocating it. Anyway, said knee injury and living two semesters in a fraternity during recovery took a toll on my physique, hence getting into cycling. I've been reading a lot about nutrition on this forum and some suggested websites but I can't quite figure out how people are getting numbers like 50/30/20, carb/protein/fat. These are obviously percentages and I think they are percent of calories. Like, 50% of daily calories should be carbohydrates, 30% protein and so on.
Someone please tell me if i'm doing this right: I have a meal shake that is 180 calories= 30g carbs, 9g protein, and 2g fat. Would this split be about 73/12/5 or am I way off? If i am right about that, then in order to get the 50/30/20 split would I need to eat more fat and protein with few carbs to balance it out?
Thanks for any help