Some use nic in a percent that divides easily for their desired final percent. Say you
vape 12 mg/ml
juice. The 48 mg nic base dilutes down easily.
But using the calculator Senior linked makes it easy as well to dilute down from, say, 100, to any concentration you like.
As mentioned, good safety precautions are important to your health. While if you mix at, say, the kitchen sink, where you can rinse your hands immediately, you certainly don't want to get any in your eyes!