I started out with the idea that a flavor should be "definite", and the juice vendors encourage that. But when I switched to subtle flavoring, several things improved, including losing that full o' something sensation in the lungs, and having much less nasal congestion. Flavorings are a bigger health unknown than the
e-liquid base. Another concern is the presence of Acetyl Propionyl or Diacetyl in far more of the composite flavors than we think, because they add a lovely smoothness. There are people who argue that these compounds are toxic regardless of amount, but that's not possible, as they are present in tiny amounts in the buttered toast and pancakes we have been whiffing since childhood,etc. But at 15% flavoring it's an entirely different story. With DIY we have several opportunities to be smarter than when we buy somebody else's proprietary e-liquid. The most obvious is to go easy on flavoring, but we quickly get adjusted to expect sledgehammer taste.
Try this experiment. Start by
vaping unflavored VG+PG+Nicotine. It's actually nice. After an hour or two switch to a DIY flavored at no more than 2.5% total, and you might be surprised to discover you can now taste all sorts of nuances that a 10% flavoring obliterates. With the little flavoring droppers like from FlavourArt, a drop is close to 25 microliters, so on a 5 ml mix that's 0.5%. I find that 4 drops total of different flavors is ample, I am breathing better, and at 2% flavoring I'm happy knowing I'm not inhaling as much of the unknowns. That's just me of course.

Cheaper too ;-) My wife says "what do you care if people want to do crazy stuff?" OK. Like I said, that's just me.
My favorite chain-vape recipe these days. Distilled from collective wisdom here.
For 5 ml of 66/33 PG/VG at 20-40 mg/ml nicotine (I like 40)
1 drop Cuban supreme from FlavourArt i.e. 0.5%
2 drops Desert Ship FlavourArt 1%
1 drop Vape Wizard FlavourArt 0.5%
5 microliters Tobacco Absolute (the 50% diluted kind) from PerfumersApprentice 0.1%
1 drop menthol in PG concentrate
This resembles some of the commercial "Mboro" type cartridge fills, but not as sticky sweet and more "open". If you like even less sweet, reverse the Cuban and DesertShip proportions. Other FA "tobacco" flavorings work well too. For a more genuinely tobacco type, use only the Cuban and TA, like 3-4 drops Cuban and 10 microliters TA, but start at 5, it's powerful stuff, and too much tastes like bitter hay.
The menthol is hardly noticeable, not like a menthol cig. It just mellows the mix. If you liked a menthol smoke, try to double then triple the menthol. To make a menthol flavoring concentrate, fill a glass dropper bottle with roughly crushed (not pulverized) menthol crystals. Add PG to fill. Shake under hot tap water and/or let stand overnight.
To measure the TA you can use an insulin syringe with 32 gauge needle. Very gently pull up a small amount from the TA bottle. Very very gently push out droplets. Each tiny droplet on mine is about 2.5 microliters, so 5 microliters is 2 droplets. You can push the remainder back into the bottle if the syringe was clean. Another method is to pre-dilute the TA 10 to 1 in Alcohol in a little glass dropper bottle, and then you would use 2 drops. Scubabatdan adds Ethyl Maltol to his pre-diluted TA.
The size of drops varies with the liquid and the "dropper", so if you want to be accurate (so you can scale up easily for biggger amounts), you HAVE to calibrate your droppers. For instance use a narrow 1 ml vial and put 1 ml liquid (measured carefully with a small syringe) in it. Mark the bottom of the liquid level that is 1 ml. Then pour it out completely and count how many drops it takes to reach the line from each dropper type. Remember to take your time so each drop breaks off on its own, not from shaking or blowing it out. Try to find some fat and some very fine droppers and mark them so you know how much a drop is on each. Using a fine needle syringe is tricky but can give you some very small drops. There's a guy selling thin tip (12 microliters/drop) pipettes on e-bay, about $6 for 50.
This recipe is a good starter for tobacco types. Just trying the various FA "tobacco" flavors in different proportions offers a lot of options. Tonight I went Cuban and CamT, and now I know CamT is not my fave, a bit "dry, bitter herbal". The TA is always important for the tobacco leaf note, menthol for smoothness, VapeWizard for roundness, Cuban for subtle tobacco richness. For another variation add a drop of e.g. a fruity or nutty or chocolate or caramel flavor. Vape on!