How much tea tree oil per 100ml of shampoo?

Some people are very sensitive to tea tree oil: it is a good idea to test by putting a small amount on a finger tip or q tip and applying it to the scalp. I use less than a drop (the amount that sticks on my finger tip when I cover the bottle with a finger and shake it) in about 2 ounces of saline in my neti pot to wash my nose when I am ill. My husband uses several drops in the same amount of water. If you wish to use tea tree oil on a daily basis in your shampoo, I would use perhaps 1/2 to 1 drop per ounce of shampoo. Make it stronger as needed and tolerated to treat scalp problems. For chronic dandruff, it would be wise to start with the weak 1/2 drop to an ounce, and to increase the strength by 1/2 to 1 drop per ounce each week until you see an improvement or it just gets too irritating for you.