The DIY technique I figured out for my friend is to do the calculation once for the nicotine you have. You just decide the nic strength you want and multiply by the size of your mixing bottle, say 10 ml, so if you want say 18mg/ml target, that is 180 mg for the whole 10 ml. Then you calculate how much of your nicotine base that is in ml. Say your nic base is 48mg/ml, you divide the 180 mg by the 48 to get how many ml of your nic base gives you that much nicotine. Conceptually you just have to go through this once. So here you want 180mg in the bottle, so you need 180/48 = 3.75 ml. Now you fill a syringe with that much water, 3.75 ml, and pour it in your mixing bottle. Mark the level with a marking pen and/or label. Cover with scotch tape so it doesn't get erased. Shake out the water. If you want, repeat the level marking for the full amount, 10 ml so you know where the full line is. With your levels marked, you're all set. Now it's super fast and easy.
When you go to mix some juice you start out by pouring in enough nic base to the first line. Then you add VG or PG in whatever ratio you fancy, I use all VG, but if you don't mind PG, use some, the exact ratio doesn't really matter, up to close to the full level, then you drip in the flavoring and maybe a few drops of alcohol or water if it's too thick. If you're still shy of the full line, add the final VG/PG. Put the top on, shake, put it under hot tap water, shake, shake and preferably let it sit overnight. Done.
Of course if you want, the same applies to a bigger bottle, or MUCH bigger bottle, but I dunno, there's something calming, therapeutic, about mixing 10 ml at a time, like meditation.