I, too, would like to know. It is so difficult when you see a recipe that says 'adjust to taste'. I understand why, but I do not have the patience to mix in drops. Further, taste testing in a drip atty does not suit me. Liquids taste much different in a clearo and carto - to me anyway.
One piece of advice I got from a retailer was this:
Flavours have a saturation point. For flavour art, this point is 5%. I was advised that 5% should be the total of all the flavours used. Not sure if that includes sweeteners. However, I tried this and I feel the sweet spot may be more than 5.
Capella and Flavour/Perfume apprentice is 20%.
Still trying to find the grail and trying to stick to those guidelines as a general rule. There are a couple of fairly large threads on this forum dealing with DIY which I found very interesting. But not a lot about mixing flavours. Usually focussed on how much of one flavour to use.
I have used Flavour Art Flash to provide throat hit. Start at one drop per 5 ml then crank it up to taste.
Another way is to buy ready to vape flavours and then mix.
As I said, I have little patience for drips and drops. The retailer has a very nice Desert Peach
tobacco flavour which is described as a blend of Desert
tobacco, peach and sweetener. I have been stubbornly trying to replicate it but failing miserably. One day ......