How much Roz Carbon and GFO to use?

[stellarfly]

Hey guys, Im sure this question has been asked many times over. However I am new to this site and I could not find the answer and I have checked on the forum. My questions are:

1. What is the method of measuring the amount of activated carbon needed in a 90 gallon reef tank. Or any sized tank for that matter? is there a calculation? like cups per gallon or something?
2. Also how much GFO? I see a lot of dual reactors, but no body states how much of each goes in. Does it even matter?

Thanks a bunch