First thing I do is look at the website where I purchased the flavoring and see if they list a suggested starting point, then look at the reviews(sometimes posters will list what they have used). Failing that, look the the manufacturer's website and see if they list suggestions(some do, some don't). Then smell the bottle....lol....see if it has a strong type of scent or faint one. This can determine how dominate the ejuice can be. If it is a stand alone flavor, you are usually pretty safe starting @5% and adjusting to taste. If it is to be the main ingredient(but not the only one) then it needs to be more than the others you add. You can also go to the website the other poster listed and see if any recipes listed appeal to you. Hope this helps.