A drop or two in a pint is good. Make sure it's pure essential oil.
Good point. There are some vendors who advertise "essential oils" that aren't selling them pure. They're diluted with vegetable oil, such as grapeseed oil.
This is more common with stuff sold for massage or aromatherapy, and is more common the more expensive the essential oil is. If the vendor says you can put the stuff direct onto the skin, its probably diluted, or they aren't fully aware of the risks of neat essential oils.
In the case of essential oils, as in life, you get what you pay for.
Tea tree is cheap, so is likely to be sold straight. Something proporting to be rose essential oil won't be, as rose absolute is is in the range of £1000's per kilo
If you get Pharmacopoea grade Tea Tree it should be the real deal. Don't spill it. It stinks, and don't get the neat stuff on your skin, its not good and will probably cause irritation (and it stinks)
An ex boss of mine dropped a 60g sample in his office and cleared a whole large production lab for the best part of an hour with that stuff
"Flatter Me, and I May Not Believe You. Criticize Me, and I May Not like You. Ignore Me, and I May Not Forgive You. Encourage Me, and I Will Not Forget You."