It depends on what amount of nic mixture you have... I would say if it's a large amount, don't flavor all of it at once, separate it out into smaller amounts, so if something "ick" happens you will not have ruined your flavor. Unless you purchased 30 ml of "nic base."
As far as flavoring it, it depends what you have. If it's a "one shot" flavor that doesn't need steeping I might start around 5% as you can shake and vape without much steeping usually. Is that what you have, though? We need more information.
Basically, it's easier to under flavor a mixture than to try and rescue it by diluting it down, so that's my general statement. If you want to mix in various different flavorings that's a different matter, and I would proceed to the DIY forum and read some links on flavors and juice calculators and ask more questions there.
Commercial flavorings are generally often in the 10--15% range. IMO, that is too much flavor for me, and it also depends on WHO manufactured your flavoring as different makers dilute it to different strengths.
You also will dilute down nic strength depending on how much flavoring(s) you add.
Best of luck, but we do need more information to not speak in generalities.
Anna