I've learned a lot from Bill's Magic Vapor in the TFA (The Flavor Apprentice) thread. He deserves all the credit, and none of the blame

The stubbornness to never use specialty juice calculators is probably just me, however.
Nicotine: 100mg/ml, 100%VG.
Goal: 70% VG liquid (turns out I really don't like PG).
This is easy to work with, when you adopt the viewpoint that
everything but the nicotine is simply diluting the nicotine.
Let's say I want 30ml juice that's 24mg/ml.
Nicotine: 2.4ml per 10ml of juice. This is because 2.4/10 = .24 = 24%, and I only want 24% of those 100 mg/ml, that is, 24 mg/ml. This means a grand total of 3*2.4 ml = 7.2 ml.
My flavorings are pure PG. Let's say my recipe calls for 20% of the total mixture to be flavorings. That means the remaining PG, 30% - 20% = 10%, must be pure PG. That is, 0.1*30 = 3ml of PG.
Now I've got flavors (0.2*30 = 6ml), nicotine (0.24*30 = 7.2 ml), and PG (0.1*30 = 3ml), for a grand total of 16.2 ml. The PG had all been added, leaving only VG, 30 - 16.2 = 13.8 ml of it, in fact. Done! No calculator needed.
I've also been brainwashed

tongue

to believe that good juice should be pretty good straight out of the tap. Shake, let it sit cap off (breathe) for an hour or two, and enjoy. Sitting for a day is recommended, but what I've made so far (fruit mixes) has been good fresh, and quite good a day later. I simply can't wait a week+, and haven't needed to!
I've also learned the magic of "100 drop tests" to avoid making 10ml+ of sub-par juice. Just use one drop for each % of your recipe, nicotine and all. Use the same dropper (or plunge the same syringe very very carefully) and it should be accurate enough to let you know what you're getting yourself into. Tweak (and do another 100 drop test) or abandon if not good enough.
Happy mixing!