For the record, we mix most of the premixed juices at 15% (less for menthol, mentholyptus and absinthe though, and 20% for some of the lighter flavours,) - but different folks have different preferences - I know a lot of folks who like 20%+, and some who prefer 10% (the hardware does make quite a difference, so you'll probably want to experiment - 15% seems about right on a standard 510, but if you're using HV or LR, perhaps crank it up to 20%+.)
That's the beauty of mixing your own, of course - you don't have to please anyone but yourself. Hope you enjoy them!