Good sound advice already been given by
@oldhippydude
Flavours & their concentration levels vary greatly
bit like orange squash concentrate & double concentrated etc...
5% to 15% is the norm perhaps but it varies immensely & everybody's different
steeping makes a difference and flavours change or perhaps "evolve" during steeping
too much flavouring can fuck up your taste buds (vapers tongue)
and some strong flavours in high measures will make your piss stink too
I'm on about 8% aprox flavouring but as said that means little depending on actual concentrates used
though I personally would call it a somewhat subtle level of flavouring imho
though I'm not a wine tasting flavour freak
(no offence to others that like an abundance of flavours cascading like a waterfall over their taste buds
or a friggin' Niagra/Tsunami of flavour)
Go easy, mix mild measures in small quantities to get a reasonable base level
then tweak/adjust from there if need to
WRITE IT ALL DOWN - seriously, write it ALL DOWN what you done
even if it is really simple & basic, you can get confused or forget the exact figures/measurements
if you fuck up, but you have the stuff/measures to hand...
if then it goes really shit, you can post your nightmare mix & what you fucked up
then somebody can work out what you "might" be able to try to salvage it
or if it is lost beyond hope and tell you to chuck it all away...
Maths was never my strong point, but I'm not thick
yet at times these calculations on nicotine & flavourings etc...
& how they alter in relation by adding more glycerine can get really confusing
think you got the gist of the basics & reckon you'll be OK
Flavourings are the most expensive part of the juice
(so another reason not to go too nutz and waste money on ultra rich & manky juice)