Save money and don't bother with using carbon at all
Fish vary in how they cope with whitespot. I have seen some really bad cases survive in the end while some seemingly mild ones drop dead.
Find the cause while you are treating. Whitespot is a sign of stress and 99% of the time it is poor, or incorrect, water quality. What size tank is it in, how long has it been set up, and what other tank mates are with it?