It doesn't have to be all sick. Every note has to be better than a bad. So you could have have a 99% with everything being sick except for 1 bad note, but a 100% with 90% sick and 10% good
to get a 100%, you have to hit all the notes sick and/or goods only, which will give you a pink p rating in the freeplay menu, you can get a golden p by only hitting just sicks, no goods. which will also give you a 100% but with a golden p.