After experimenting a bit - I have found that, for me, the simplest is the best.
Normally I smoke 24mg/ml. I have 60mg/ml nic which means that I need to add 1.5 ml of pg or vg + flavorings to every ml of nic juice to get the correct mix.
I use a syringe to measure; measuring directly into the bottle I will be using. It thus only takes a minute.
I like my flavors fairly strong about 20% favoring. I usually mix 10mg at a time. So, I just fill the 10mg dropper bottle I am going to keep my e-juice in with 4mg nic, add 2-3mg of flavorings and then fill the rest of the bottle with 3-4mg pg or vg.
Of course, it did take me awhile to figure out the flavors I like and which favors go well together. For a couple of my favorites I just use one flavoring. I love Lorann's Butter Rum and Chocolate Hazelnut and just mix them alone with a couple of drops of ez-sweetz. Others are a bit more complicated like 1mg PA's peach + 1/4 mg Lorann Orange Cream + 1/4mg PA coconut and 1/4mg PA's Pineapple and 1/2mg PA's French Vanilla + ez-sweetz , but even that one takes less time to mix then to type out.