Hi. I'd suggest reading through these guides to get a general idea how this is usually done:
http://www.ecigexpress.com/blog/diy-how-to-for-dummies/
A Beginners Guide to DIY Electronic Cigarette E-Liquid | Vaping Guides to the Electronic Cigarette
E-Cigarette Forum - Hoosier - Blogs
It sounds as if you got some flavored 0mg/ml nicotine e-liquid, and want to add unflavored nicotine e-liquid to it. That will not only change the balance of PG/VG, as Levitas mentioned, but it will also lower the flavor level of the e-liquid that you have bought.
That's not quite how most people here do DIY e-liquid, but I'll get to that later. If you're looking to get supplies from HealthCabin, you may want to get unflavored nicotine e-liquid in the 36mg/ml nicotine strength, and bigger size bottle, as it's more economical than the lower strength and smaller sizes.
In order to keep near the 70PG/30VG ratio that you already have ordered, you could get the 36mg/ml nicotine 75PG/25VG unflavored e-liquid. There are DIY e-liquid calculators listed in the guides that I posted above that can help with the math, but it's not very hard to do by hand, most of the time.
You have 10ml of 0mg/ml flavored e-liquid, and would add some amount of 36mg/ml to achieve a 6mg/ml result. Since 36 / 6 = 6, then if 1/6 of your mix is the 36mg/ml, and 5/6 is the 0mg/ml, you will get a result of 6mg/ml.
Another way to look at that is to use a formula for averaging all of the 6 parts:
(36 + 0 + 0 + 0 + 0 + 0) / 6 parts = 6mg/ml.
So, if you have 10ml of 0mg/ml, you'd need to add 2ml of 36mg/ml to get 12ml of 6mg/ml liquid. The flavor would be reduced by 1/6, though.
Yet another way to look at it, which also illustrates what mg/ml actually means, is to total the milligrams of nicotine added, and divide it by the total milliliters of liquid.
10ml of 0mg/ml liquid has 0mg of nicotine.
2ml of 36mg/ml liquid has 72mg of nicotine dissolved into 2ml of liquid. 72mg / 2ml = 36mg/ml.
Mix those, and you have 72mg of nicotine in 12ml of liquid, and 72mg / 12ml = 6mg per milliliter.
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Now, that answers the original question, but I wouldn't really recommend doing it that way if you want to get the most out of DIY. Normally, you'd buy unflavored nicotine liquid, plain PG and/or VG, and e-liquid flavorings (food flavorings). Syringes to measure with, and empty bottles to work with would usually be the minimum hardware to buy. You should plan your purchase of the liquids for your target PG/VG ratio, but it wouldn't hurt to buy some of each plain PG and VG so that you can be more flexible with the result. Again, if HealthCabin is the vendor that you want to use, this is what I would get:
115ml HC Unflavored E-liquid (36mg/ml, 100% PG)
115ml PROPYLENE GLYCOL USP
115ml Vegetable Glycerin USP
Some 10ml bottles of flavorings that sound good to you
Empty Ejuice Bottle(childproof) - several of whatever sizes you like; 10 and 30ml are my favorites.
E-juice Injector(1ml 2.5ml 5ml 10ml 20ml 30ml) - a couple of 5 and 10ml sizes is usually fine.
The flavorings have to be experimented with in order to find the amount to add of each to get the desired flavoring level. It's best to start with single flavor mixes, and note how much flavoring you used to get a good flavor, before trying to mix more than one flavor.
This is the hardest part of DIY, and takes some time to master. Hoosier's blog link near the top of my post explains that well.
There are many ways to get a 70pg/30vg 6mg/ml result. The supplies that I listed aren't the easiest way, but are lower overall cost, and easy enough. Let's say that you have found that the flavoring that you want to use is good at 5% of the mix. Here are the ratios to make a batch of any size:
16.67% unflavored 100% PG 36mg/ml nicotine liquid (1/6 of 36 is your target of 6, so 1/6 of the mix is needed)
5% flavoring (these are generally PG-based or alcohol-based)
48.33% plain PG (this should be your 70% PG target, minus the 16.67% PG nic liquid, minus the PG flavoring percentage)
30% plain VG
So, for a 10ml batch:
1.667ml of unflavored PG 36mg/ml nicotine (1.667ml * 36mg = 60mg in 10ml total = 6mg/ml strength)
0.5ml of flavoring
4.833ml of plain PG
3ml of plain VG
While you can't measure quite that accurately with a syringe, you can come close enough that it would be fine. Also, note that you'd run out of PG before you would VG, since more PG is used. You'd run out of both plain PG and VG well before you ran out of the nicotine liquid.
I hope this helps you to understand. Feel free to ask questions if you don't understand something.
