For Redneckhippie, so I can tell you how I would do this but it's not simple. If it totally doesn't make any sense I can type it up using an equation editor tomorrow morning and make it look clearer on the page. I'm going to step through an example using easy numbers so it is clearer. For this example your going to make 10mL of juice that is 20mg/mL nic and has 50%PG and 50%VG as the final mix.
You have unflavored nic at 60mg/mL. To find out how much of the nic liquid to use, divide the desired 20mg by 60mg to get .33 or 1/3. One third of 10mL (target amount) is 3.3mL so that is how much nic juice you want to use for 10mL at 20mg/mL.
You are using a 50/50 nic mix so 1.65 of the 3.3mL is PG and 1.65 is VG. I keep a running sum. Write one column for PG and one for VG. each column has 1.65 in it now. In the end both the PG and the VG column should add to 5 so that 5mL of VG plus 5mL of PG will equal 10mL of juice.
Now for flavoring. Most Flavoring is PG based, so whatever percentage of flavoring you are going to use put a decimal point in front of it and multiply that times 10. So for this case I am going to use 20% flavoring in my juice. 20% of 10 is (.2*10=2) 2. So 2ml of flavoring goes into the mix, and 2mL is added to the PG column(assuming that it is PG based flavoring).
Now my PG column is 3.65 and my VG column is 1.65. All I have to do is subtract 3.65 from 5 to find the amount of additional PG that I need and subtract 1.65 from 5 to find out how much VG I need to add.
5.00 - 3.65 = 1.35 mL PG 5.00 - 1.65 = 3.35 mL VG
Add those to the mix and your done.
Now I used 5 because it is 50% of 10. If you were doing a 60PG/40VG mix ( as a final product) it would be
6.00-3.65 =2.35 4.00-1.65=2.35
because 6 is 60 percent of 10 and 4 is 40% of 10.
If you want to add distilled water or PGA to thin it that will change the calculations a little. If I want to add 10% water in this instance, 10% of 10 is 1. So instead of my columns needing to sum to 5mL each they now need to sum to 4.5 mL each. You don't have to use any distilled water or PGA if you don't want to. If you are using a lot of VG, the juice will be pretty thick and might not wick fast enough.
Hope this helps.