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Use ATF.
That said, with 15% flavor, your actual nic content with 70/30 3mg/ml or 70/30 6mg/ml is going to be 2.55mg/ml or 5.1mg/ml respectively, and the final ratio will be 59.5/40.5 for each.
Also, make sure you check the documentation provided with the premix so you know what to weigh out.
Here's my favorite calculator.
http://www.ejuice.breaktru.com/
+1 for this.
One of the very few calculators out there that let you save your recipes. Very easy to use, reliable, no internet required after you download it so you don't have to worry about server issues (looking at you ATF), completely free.
A quick guide into actual DIY:
You use basically 4 things: VG, PG, nicotine and flavours.
In reality though even nicotine and flavours are basically PG with a few chemicals thrown in them. (there are also offered in VG or 50/50, but usually they are PG).
So if you hear a juice is 70/30, that could be 70% VG, 10% PG, 10% Nic, 10% flavour. (These number are for demonstration, use recipes).
Also keep in mind that nicotine is sold in bottles that say their concentration in mg/ml. In my country the law says that nic should be sold in maximum 20mg/ml (😐), so let's use that.
So if you want your juice to end up at "4mg" that means that your juice should be 20% nic, 80% all your other stuff.
So if you want 100ml of 60/40 juice, with 10% flavour and 4mg strength you do put in:
-60ml of pure VG -10ml of PG Flavour -20ml of 20mg/ml PG Nicotine -10ml of pure PG
Wear gloves for the nicotine, shake it well after you put everything in.
Get accustomed with a calculator so that you don't do math everytime (and not do any mistakes accidentally). I use the e-liquid-recipes one, but I haven't use anything else, maybe there are better ones out there.
Use this http://e-liquid-recipes.com/create instead of steam engine. On the make a recipe page just select use vape ready nicotine base. Make sure you input the pg/vg and nicotine concentration.
Just be careful when researching flavor percentages on here as the recommended % can be way,way off.
Theres an app called liquid calc that over simplifies using a base for mixing
I hope this is the app you're talking abt.
http://imgur.com/kcLgTlU
Anyways did i denter everything right? I was planning on using a 70/30 3mg/mL nic premixed base. Then I was going to use nicotine itself to bump the the final nic lvl to 5mg/mL
With 15% pg flavor, the finished juice will be 58/42.
Just wanted to mention that it might be easier and last longer to skip the premix altogether if your planning on getting a nicotine anyway. You're having to add extra steps and extra math by using the two bases. Not sure which nic base you're planning on using to bump it up, but it will likely last much longer than your premix at 3 mg/ml, whether you use it as you described or skipping the premix altogether. I would recommend getting the 30 ml 100 mg/ml nicotine from ecig express if you decide to skip the premix. It usually lasts me about a month and I chain vape. I think you'll see you get more bang for your buck that way and it will make it so much easier to use the calculators. Here's a simple calculator http://www.todmuller.com/ejuice/ejuice.php
Just do a dry run with it and you'll see what I mean by skipping the premix. Also, you can buy a 120 ml bottle of VG from walmart pharmacy, usually found by the bandaids.
in your case, it would just be a matter of adding in flavor. 10% flavor is a good starting point, so if using a 30mg bottle, I would add 3mg of flavor, fill the rest of the bottle with your premix, and shake and vape. 1mg flav for a 10 mg bottle, 2 for 20, 5 for 50, etc. Adjust to taste preference.
A lot of manufacturers have a recommend starting point for their flavors on the bottle. in those cases, the equation is basically convert the recommended % to a decimal and multiply by your bottle size to get the mg's of flavor.