The hardest part of mixing your own is getting the end PG/VG ratio correct. To do that, you need to know what the nicotine base carrier is (usually all PG, but not always) and the ratio of PG:VG in the flavoring. When you have how much juice you want to make and the ending nicotine content - you can plug all that into one of the calculators mentioned. The calculators even allow the use of multiple flavorings.
But, I am lazy. Since the percentages of base and flavoring are generally relatively small, I make it easier by ignoring the PG:VG ratios in those. I also vape at approximately 50:50 PG:VG so premix the main dilutant to that ratio.
As an example, say I desire 100 ml of juice and my nicotine base is 100 mg/ml (1 mg/ml). Also assume that I want the nicotine content to be 3 mg/ml and I will be using 10% of flavoring by volume. The simplified calculations would go like this:
Since I want 100 ml of 3 mg/ml juice, I need 300 mg of nicotine or 3 ml of the nicotine base. The flavoring is 10% of the total volume or 10 ml.
The two add up to 13 ml - so I need 87 ml of my 50:50 PG:VG dilutant.
That is all there is to it. As I mentioned earlier, the PG/VG ratio will not be exact. It will be somewhat PG heavy, but it is close enough for most people.
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To get it much closer takes a few more calculations. First, lets assume the nicotine base and flavoring are both in PG carrier. So the 100 mg/ml base is very close to 90% carrier. 90% of 3 ml = 2.7 ml of PG is in the base and you can assume almost all the carrier in the flavoring is PG (10 ml).
So there are 10 + 2.7 = 12.7 ml of PG in the mix before adding the dilutant. So for an ending 50:50 juice - add 12.7 ml of VG to the mix. That gets the ratio to 50:50 for the submix.
Since you needed to add 87 ml dilutant and you already added 12.7 ml of VG, you need to add 87 - 12.7 = 74.3 ml of the 50:50 dilutant. Or, just bring the volume in your graduated cylinder up to 100 ml volume.
