I must confess - I cheat a little. I read reviews on any flavor I'm unfamiliar with, to get a feel for how other people like it, and lots of times, they'll say how much they used in a mix. It gives me a little headstart that way. That's how I found out how to love TFA's caramel cappuccino.
Someone's review said to use ONE DROP per 10ml of finished juice. All the other people who said it resembled skunk spray were presumably using it in percentages (3%, 5%, etc.) which would be WAY too much for that flavor. So I plucked up my courage and ordered a small bottle of caramel cappuccino, used it at the one drop per 10ml ratio, and it's wonderful.
So read reviews on the flavor sites if they have them (Bull City does), do a search for that flavor in the DIY sections to see if there are any discussions about the particular flavor you're considering, and in general try to learn as much from other people's trials and tribulations before you commit your own.
If you can't find any information on a flavor, ask here. SOMEONE will have experience with it, no doubt. You can save a lot of headaches that way, and really, most people are happy to share their experiences with flavors. We all want you to succeed!
I have very few recipes that use ingredients in equal amounts, for instance. I do love Nonna's Cake, and I use that one a lot, usually at 4%. I've never used honey so I can't comment on that flavor, but any nut I might add would be in the .5 to 1% range. I wouldn't sully Nonna's Cake with espresso at all. I use espresso (by FA) at 1% to boost the coffee flavor in my cappuccino juices. Nonna's Cake is VERY good with cream and custard flavors, or with a very light touch of fruit. I still get that wonderful flavor of angel food cake but the custard and cream flavors add lusciousness and depth to the overall flavor, and sometimes a touch of fruit brightens it up. If I wanted some coffee with that lovely cake, I'd have that in a second tank and take turns vaping them.
If you really want a coffee flavor added to your cake flavor, try a cappuccino-type flavor, but read the reviews first! LOL! I just got some Capella's cappuccino which is very nice at 3% (strong enough for me, a true coffee-holic), but of course I still soften it with other cream flavors. And the TFA version is way stronger. I used to use the Flavour Art cappuccino, but it was almost too soft for me - I found I was having to use it at 10% and it's too expensive to use that much of, so I went looking for alternatives. I think I can safely say I like the Capella's cappuccino better than the Flavour Art, it's cheaper, and I don't have to use nearly as much.
Taste is subjective so what works for me might not work for you, but if you read enough opinions, you can sift through and get a better idea of how a certain flavor might taste, and how it might work with other flavors. So ask away, we're here to help!