If you enter the high values of the range into the "Target" column, the calculator uses those values to suggest chems and quantities.
The range you're seeing comes from how you set up your goals. If you enter numbers in the target column outside of those ranges, the calculator will still use those numbers to recommend quantities to get you to that particular value, same with lower numbers. Basically you control what you want the calculator to calculate for you and the ranges are listed as guidelines.
Edit: The one value I would suggest always aiming for is the high end of the chlorine range. You'll aim for this target number in the evening after swimming and sun are over. It will help you stay within the range for the next day, hopefully never falling below the minimum level for your CYA level. The next evening, you dose it back up to the high number again, and so on.
