Here is something I found about it:
Up to 2% or 2 drops per 5 mil should be ok.
"Smooth" will round out any harsh edges and provide a thick, satisfying mouthfeel. It will soften the high notes of your blend while boosting your background notes.
It can help some mixes produce more vapor. (Some flavorings actually reduce vapor production) It can smooth or help blend flavors together. I use it in the 1 drop per 10ml to 1 drop per 3ml range, but usually start closer to the middle at 1 drop per 5ml to see if it will do a darn thing to the mix. Sometimes it seems to work wonders, but I've never had it make a mix worse.
As an addition to other flavors 1-3%
Work your way up:
10 ML, start with 0.1 ML
30 ML, start with 0.3 ML
Contains Triacetin.
Acting as a flavor enhancer that is useful primarily in concert with other flavors, Smooth "rounds out" harshness, softening strong flavors and enchaning background aromas. Smooth has a natural fruity hint which mixes well with other fruits or caramels. A flavor smoothener that is useful to use in concert with other flavors. "Smooth" will round out any harsh edges and provide a thick, satisfying mouth feel. It will soften the high notes of your blend while boosting your background notes.