Use the calculators to find out how to get to where you need to be. Also, find a volume calculator to find out how much actuall water you have in your system. I have 90g and a 60g sump/fuge.... but after figuring actual water levels and live rock and sand, I have a total water volume of 110g. That is the number I use for all my calculations. You may have 45, or you may have 35. But it does matter.