Activated charcoal is very effective in absorbing all kinds of chemicals and therefore a good marketing argument since most people know this. I cannot see how this will work in a shampoo though. The charcoal will just “grab” the the available ingredients in the shampoo formulation and get “deactivated”. For marketing reasons use as little as possible without loosing the trust of your customers (who are buying it because they think it works). 0,5% is a good level for this purpose.