I agree a calculator till you get the hang of it better would help you greatly....not only is your nic base a pg or a vg base or a mix of both but what strength is it? do you know if your flavoring is PG or VG based or something else like alcohol based? When mixing DIY these are things you need to know because they effect your PG/VG ratio and a calculator would greatly make doing the numbers much easier helping your first mixes turn out better. You have to remember that when you mix up a fresh bottle of liquid that the flavor is going to change and steep over time....especially the first two weeks the flavor can change drastically and what might taste good the first day may be too weak or too strong flavor wise a couple weeks down the road and some calculators like "E-juice Me Up" (google that name and it will come up, it's a free download and comes with full support.) allows you to save each mix you make as a "recipe card" and has not only calculators for all your flavors and bases but a section for notes so you can keep track of everything on your recipe card so if you want to change something you can save each variation of your mix as a different card and keep track of everything with relative ease.
DIY can seem a little overwheming at first but if you do a little reading on it around the DIY part of the forum to get the basics and get a good calculator to crunch the numbers for you so you don't put too much nic in or too much/little flavor till you get the hang of it with a few bottles it can be quite a rewarding hobby....some of the best and most rewarding vapes I've ever had were from a 2 week steep mix of my Banana Bread recipe on a fresh parallel coil in my Magma. There's something about making it all for yourself, your juice, your coil, that make it much more rewarding.
Untill you get the hang of mixing and diluting your mixes properly I would strongly suggest that you buy your nic prediluted....so if you want a six mg nicotine level in your DIY by 12 mg nic base or even 6 mg. If you by the same strength base as you want your juice to turn out it will be slightly weaker than if you had diluted a stronger base, that is true. But in the trade you only have to order your desired strength and pg/vg ratio (most nic suppliers sell 30pg/70vg pre-diluted and mixed that way in stock,) then you only have to had your flavor and you have instant e-liquid. By buying the next step up (buying 12 mg nic base if you want 6 mg liquid,) then you have to still dilute it, but it's much easier than if you were trying to dilute down 24 or 48 or even 100mg nic base which can be very dangerious to handle if you don't take the proper precautions of wearing gloves and long sleeves so it doesn't come into contact with your skin and a mask or having a window open and a well ventilated room to work in so you don't get nic poisoning from the fumes...bottom line, start with the desired level of nic for safety till you get your skills up...it won't take long...and pre-diluted nic is much cheaper too.
Another good thing to know about DIY that a lot of people don't know when they first start because people don't always know or tell others, I didn't know this till I had been mixing for several months and figured it out and researched it to confirm this, is that VG not only is not an ideal carrier for flavorings, but it is not an ideal carrier for nicotine either. If you buy a nic base that is either in 100 percent or more than 50 percent VG you need to constantly shake, and I mean really shake, up that bottle as hard as you can for as long as you can. Nicotine can settle and separate from VG over time from sitting still so if you don't constantly keep mixing the two together by stirring them or shaking the bottle up really good at least once a day it can separate and you can stick your dripper or syringe into the bottle close to the top/bottom and get less nicotine from it than you're supposed to or way more nic than you're supposed to and that can make you sick....bottom line....always keep your VG well mixed. PG based nic needs to be shook up and mixed regularly as well, but it blends much easier and keeps much better and is far less likely to separate if it does at all.
To get your desired 30PG/70VG mix I would by your nic base in pure PG as well as your flavorings PG based so you have better flavors and more evenly and easily mixed nic and then just and plain 0 nic VG base as needed to get the desired results. Also, if you don't have one I would strongly advise you getting a nicotine test kit so you can test the level of nic in your mixes and make sure it's right. It's extremely rare that you get the wrong strength of nicotine base, it's almost always within 1 percent of what you ordered...but mixups have happened in the past and it's better to be safe than sorry....you can get a really good basic kit at the web link below....better to be safe and test your first few mixes before you just mix up large batches and waste supplies because it's too strong or not tasting right. Better safe than sick
http://www.eliquidtest.com/
Please...if you have more questions ask them...we are here to help and we want to help you vape safe and vape happy...I am sure as you start to mix you'll have questions about other things...we're here if you need us bud. I hope this info helps, I know it's a lot to take in but once you get the hang of the basics it will feel more and more simple over time. Here's a basic formula for 10 ml's of 6mg eliquid made with 12 mg nic base...you'd just triple it to get a single flavor 30 mg eliquid in 6 mg strength in your desired pg/vg ratio.
.5 ml 100 percent PG 12mg nic
1 ml 100 percent PG flavoring
1.5 ml PG base
7 ml VG base
that will make a 6mg single flavor eliquid...but you'd have to adjust the flavoring amount to taste and like I said, it can change drastically over the first two weeks and then it will settle and change slowly if at all. Also, one last thing...in terms of measuring....20 "drops" of liquid is considered to be roughly 1 ml of liquid.
Best of luck to you bud and safe mixing. Cheers