Also, like others have said, DIY juice doen't have to be scientific. Take a little time to mix up small batches of flavor individually, at a certain drops/ml ratio, say if it's a 5 ml batch, use either 5 or 10 drops of flavor, and get to know how strong they are by themselves, then use that experience to judge how many drops/ml to use when mixing other flavors together. Some ppls recipes are so complicated, mine are never so. Takes me about 15-20 minutes to whip up 5 or 6 new batches.
Also, if you are starting from concentrated nic, mix up a base; meaning mix up your pg, vg, and concentrated nic in a very large amount first to your desired strength, then fill your small bottle batches from that pre-mixed base to add your flavors to. They won't have to steep near as long and it's safer that way, avoiding concentrated nic pockets floating around in very thick vg, tl;dr. Trust me.