Hi @ Mrdazjones there are no hard and fast rules with flavour percentages. The highest I generally go is around 20%, if was using a lot more than that I would start to question the quality of the concentrates I was working with. Out of interest what concentrates are you mixing with?
Most concentrates are PG based, so it depends if you are working with PG or VG based nicotine and what strength nicotine you are using and also what PG/VG ratio you are aiming at as to whether a high percentage of PG based concentrate will affect the desired result. For example if you were trying to mix a 12mg, 50% VG, 50% PG e-juice and you were using anything less than a 60mg PG nicotine base with anything more than 30% PG based flavour it would be impossible to mix.
Sorry for the long winded reply and I hope it made sense. In short, use whatever tastes good to you.