I agree with Stosh - this is how I do it too.
As soon as I get my 100mG nic, I dilute it down to approximately where I want to end up. You want something between 12mG and 18mG and 70PG/30VG, correct? And you bought 30mL of 100mG nic, right? If you dilute it 1:6 total, you'll end up with 16.66mG (in your range). In other words, for every 1 part (30mL) 100mG nic, add 5 parts (5 times 30 or 150mL) PG/VG giving you a total of 180mL 16.66mG nicotine base.
Is your 100mG nicotine PG or VG based?
If it's PG based, add 2 parts (60mL) VG and 3 parts (90mL) PG. If it's VG based, add 1 part (30mL) VG and 4 parts (120mL) PG. In both cases, you'll end up with 180mL 67PG/33VG nicotine base.
That just needs to be done once. Now simply work with the nicotine base and add flavorings.
As you add flavorings, the numbers will change slightly. Most flavorings are PG based so (depending on how much flavoring you add), the PG/VG ratio will increase somewhat while the nicotine percentage will fall slightly.
Personally it doesn't matter to me if the juice ends up at 16mG or 15mG or 14mG, nor do I really worry if it's exactly 70/30 or 67/33 or 75/25. I know I'll be in the right ballpark. Take notes on what you do. Then, when you find something you really really like, you can recreate it.