I use the dropper from one of my 10ml bottles and add a few drops with that. I'll then drip on the atomiser and give it a taste. If it's too weak then I'll add some more.
The last batch I made took 16 drops to get the perfect strength apple flavour that I wanted for the 9ml I made (6 ml TW Marlboro, 2 ml glycerin, LorAnn apple flavour).