OK, many recipes run around 20% flavoring. If you use that, then 20% of your final product will be flavoring without nicotine, so your 24 mg will be cut to roughly 19 mg, which sounds like where you want to end up. Then you wouldn't have to add any more pg or vg. You can, of course, use less flavoring and end up with a higher nicotine level, or add the additional pg or vg to lower it. How much flavor you add depends on whether you're doing a simple one-flavor mix (then you'll usually have a lower level of flavoring) or a complex blend of flavorings. As Frenchfry said, check out the DIY forum, especially the TFA flavoring thread - there's a lot of really helpful people there who can give you much more detailed guidance. Good luck!