They have one other flavor that's even stronger: Caramel Cappuccino. Many use that at 1 drop per 10ml; I like a drop per 7-8 ml.
Koolada...1 drop in 10ml will make your Cola a little crisper but won't be cold. 1 drop in 5ml will make your Cola cool or wake up a tired mint. 1 drop in 3ml will make your Cola a popcicle or your dull mint into a breath mint. 1 drop per 2ml is Cola in liquid nitrogen or mint like a mouthful of Altoids - takes your breath away. More than that sends most people into coughing fits. You can always add more, but you can't take it out once you've added it.
TFA's other flavors are not as strong as these two. Some are commonly used at 15%. There's a
thread around here somewhere with TFA starting percent recommendations for various flavors. It's generally safe to start about 5% and work your way up; start around 2% with most of their coffee, tea, tobaccos.