Tough call, but one of the great things about DIY is no matter how gruesome a juice is *at any given moment*, technically, it's always a "work in progress". So, two or three drops on the atty, vape a bit, savor the flavor, be violently ill (if necessary), and adjust the juice accordingly. (It's a good idea to do this in small batches, keeping track of each adjustment so if it turns into something worth replicating, you'll be able to.)
That said, I almost killed myself with 'way too much Capella's espresso flavor concentrate less than a week ago. Stuff was so stout I couldn't even taste my coffee over it, and I make my coffee strong.
Happy Vaping!