I've been making my own DIY juice for awhile now, so will try to offer some advise and a few tips..
First off, use the calculator! When you want to make something like a 15 ml bottle 8mg juice, you don't want to try doing the math yourself.
Get yourself a notebook, and write down everything! You don't want to play the guessing game or try to remember exactly how you made something last time. Most of us will start low on flavor, then add as needed, so you will want to keep track of it every time you make a change.
Flavors are not all created equal! Write down all of your flavors, and what brand they are (Loran's, TFA, etc..) make a test batch of 1-3ml 0nic for each flavor, start low, around 1% and slowly work your way up until you get the right amount of flavor, then write it down!
Some flavors are VERY strong and anything more than 1-2% is overpowering, other flavors can go anywhere from 10-20% and still be great
If you are just working with 1 flavor, it is pretty easy to figure out where the sweet spot is, but when working with 2-6 flavors in some of the more complicated mixes, you REALLY NEED to know exactly how strong each flavor is.
You should also keep in mind that some flavors really compliment each other, while others will ruin the batch and taste nothing like you were expecting.
Look up juice recipes online, study them, look at the flavors used, and how much of each flavor is used, this will give you a good idea of how to mix them and in what %'s. you will start to notice some flavors use very little, while other flavors make up most of the mix, even if it doesn't look like it should.
Above all, be patient, over time you will get better and faster at making your own juice, then perfecting it after some fine tuning.
Last, and most important, go to the DIY juice area of the forums and read EVERYTHING, flavor safty, mixing ratios, tips and tricks, what not to do, and so on.