It depends on who manufactures the flavours. I see WL has super concentrates, I've not used it so can't say. For regular (not super concentrate) flavouring my general rule is 5-10% on an untested flavour.
Do a search for recipes, there's a lot out there and you'll get a general feel for %s even if you don't use one.
The easiest thing to do, really, is to get your feet wet and experiment. Start at a lower %, let things steep a bit, try em out, make notes, adjust, steep a little, try, notes. DIY is a process that takes time, even on single flavour mixes, to get right. If you start low you'll probably end up with something vapeable early on.
Don't forget about steeping, especially with tobaccos (So I've read). A week of steeping makes a difference in flavour, some mixes need several weeks to shine.
And take notes. Always take notes.
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