Hoosiers blog is a good place to start
But you got to find it yourself, a very easy way to test out the strength of a flavor is. Get like 5ml of water, (if you want to go with drops), give it a drop, put your finger in it, and taste it. Nothing, add another or 2. This gives you roughly a good idea of how the taste would be.
Do this at your own risk. And tobacco's might not really work this way. Never tried tobacco's tbh doing it like this. But this works for drinks/fruits etc
ALso you have vendors where they advise a ratio. Usually it's something close. Some other do not, but from the same vendor there might be a 5%-15% difference in flavoring that you need