The percentages apply to your total yearly allotment. Also, you can't go beyond the total percentage in your banking. I'll try to do an example.
You have 200 points. In January, you could bank up to the full 200 points. You decide to bank 100 points. You are now finished banking for the year. The reason is because between Jan and April you would be allowed to bank up to 50% of your total points which is 100 points. You've already done that.
If you banked 25 points in January, you could bank up to an additional 75 points by the end of April because you would not be over the 100 points.