Your child's head just needs to hit the bar. That is it. However, even if a piece of paper can fit between your child's head and the bar (note it is their head, and not their hair!) they will turn you away.
This is the experience we had with our DS 6. In the fall he was measured at 48 inches at Kings Dominion but I thought the person was being a bit generous. On our trip this past December he was measured at RnRC and the Cast Member must have measured him at least 3-4 times and it was barely a papers width between his head and the bar but she turned him away. She was flattening out his hair (which is not very poofy to begin with), having him stand up as straight as he could, etc. She felt really bad about it but we told her it was ok. We noticed he was in his sandals and when we went back that night he had changed into his sneakers and we tried one more time and he got measured again and this time was told he was good to go.
On a funnier note, we went to AK the next day and DS wanted to try Primeval Whirl, so we went and he got measured. Again he was just barely let through, and told he may get measured again at the ride itself. When the CM was deciding whether or not to let him through, my DS 8 asked me why his brother was getting checked again when he was let onto RnRC the night before. The CM looked at me and told me RnRC was not a 48 inch ride. I just smiled becuase she had not okayed my little one to go on yet, but I thought it was funny. My older son wanted to tell her that it was 48inches to get onto RnRC but I just shushed him.