Hey vlad, go check out e-liquid-recipes, you can search each concentrate flavour and see the average-% people use or search for mixes. Pump that into an e juice calculator and your ready to go.
Best way really is to test them yourself. Each flavour concentrate at different percentage brings out different characteristics of the flavour. I usually check the average then test out the flavour at a higher and lesser amount and see how it tastes to get what I want.
You can also check depending on the brand of ejuice, I know FA uses a stronger concentrate so a much lower %, others can vary a lot.
Good luck
Edit checked out the percentages,
Pom Fw - 5% average, couldn't find menthol delight anywhere but start around 2-3%.