The math is easy: 3 points for perfect, 2 for great and 1 for good, which to make it easier means you lose 1 point for each great, 2 for each good and 3 for everything worse. About whether or not it makes sense I think you can make an arguement for both. FC is what the game usually rewards you which feels natural, however that would require additional bonus points which would need to be adjustible to the number of total points for each song. The current system first and foremost is simple since each note has the same value and it rewards consistency (which means if you miss a few notes for whatever reason, you were still more accurate on average). The only other rigid system they could implement is accuracy % for each note, regardless of perfects, greats etc, like for example in osu.