With tobacco flavors, you might want to start at 5% and work your way up from there. I suggest making two small sample batches of a flavor you're preparing to try, one at 5%, the other at 7%. After steeping for a few days, compare one to the other to see which you prefer. If the 7% tastes better, try 8% next time.
I've found that the tobaccos generally improve with extended ageing of weeks or even months. Put away the bottles which have a harsh or 'off' taste and try them again after some time has passed. You could be pleasantly surprised by the change in flavor. You can also improve sub-par tobacco mixes by adding other flavors such as Cotton Candy (ethyl maltol), menthol, coconut, cinnamon, marshmallow and others.