From Chefsvapour.co.uk where I buy my concentrates:
"Flavour concentrate strengths vary depending on which company make them.
FlavorWest aka FW are known for having some weak concentrates, this doesn't make them a poor choice (FW Double apple tastes just like the real thing!) it's just you may find with some that you need a higher percentage say 20% and therefore it limits you to 1 or 2 flavourings per 10ml but, what I personally find to be a weak flavour, you may find it strong. Again, it's all down to personal taste.
FlavourArt aka FA and Decadent Vapours aka DV make some really strong concentrates and some you only need to use between 1% and 5%.
Most of the FA tobacco concentrates I would recommend starting at 2% because they are very strong and they really come alive after a 2 week steep (some can take longer!) Some of the more general flavours need much higher i.e 10% or more.
Mom & Pops aka M&P make some great standalone flavours and are great mixed between 10% and 20% but this would be down to your personal taste. Again, these really do benefit from a decent steep i.e 2-3 weeks.
T-Juice also make some great standalone flavour concentrates. They are a great beginners concentrate because you can get great results from only using one concentrate. Use anything from 5% to 25% depending on your taste.
The Perfumers Apprentice aka TPA or PA have some amazing concentrates but they all vary in strength. We have used PA from 1% up to 10% depending on the type of juice you're making.
Inawera, LorAnns and Tasty Puff are all extremely strong and I personally wouldn't go any higher than 5% but again it's totally down to personal taste
Capella is any DIY mixers staple. I think every DIY mixer will have at least one Capella concentrate in their mixing box. They can be hit and miss but overall they're a great product. Use anything between 5% and 20%.
One on One flavourings are new to us and we're still playing with them ourselves but from what we've found so far they can be quite strong so use anything between 5% and 15%
Here is a usage guideline for our selection of flavour concentrates...
FlavourArt - 1% to 10%
Their Tobacco concentrates are extremely strong.
Decadent Vapours - 1% to 15%
Mom & Pops - 10% to 20%
Perfumers Apprentice - 1% to 10%
Capella - 5% to 20%
One to One - 5% to 15%
Inawera - 1% to 5%
Tasty Puff - 1% to 5%
LorAnns - 1% - 5%
FlavorWest - 10% to 20%
These percentages are only guidelines and not set in stone. Making your own juice is a personal experience so tweak them to your taste. If you're following a recipe you've found online and think you might find some of the flavourings a bit high or a bit low then change them. No one is right when it comes to percentages. It's down to you to decide whether you like Capella Vanilla Custard at 10% or 20% or whether you like more VG and less PG.
It's personal preference and it's your juice, make it your own.
I hope this may be of help to some of you but like I said, it's personal preference. Just keep experimenting it does take time to get a really nice juice but it's well worth it in the end.
Happy Mixing!"