You'll likely be a little "high" on nic (lol).
For instance:
To try a mix that's 10% flavor, you'd have say 9 mls of your nic base + 1 ml of flavor.
That's 4x9 = 36 mg of nic in 10 mls total, for 3.6mg/ml
To try a mix that's 20% flavor, you'd have say 8 mls of your nic base + 2 mls of flavor.
That's 4x8 = 32 mg of nic in 10 mls total, for 3.2mg/ml
You can pretty easily get it exact though - starting with either the 4 mg base, or starting with the raw materials, and using one of the e-juice calculators. I like ejuicemeup - if you're working with a PC, it's awesome. For an app or an apple, you'd need a different one...
So this is my approach to this whole DIY business. I want to make ~3mg max vg juice for my dripper. I understand it takes time to figure out recipes and get the flavor right. My plan is to take a gallon of USP VG and add nicotine vg base turn it into 4mg unflavored juice. Then start making recipes from this supply. I understand that as I add flavoring the mg/ml will drop and that's the reason I'm starting at 4mg. Should I start higher than this if I want ~3mg after flavoring and is this a good approach to take just starting out?