I tried this at 2 drops per 10 ml and it was overwhelming. 1 drop per 10 ml is perfect. Whatever you do, don't go up to like 10 or 20 percent, I thought I'd never live long enough to tell the tale!
By the way, this is one of my favorite vapes, with about 5 percent sweet cream and a tiny dab of sweetener if needed. Perfection!