I mix from scratch and from premade bases. I totally agree that adding a premade base to concentrates is a real problem solver, if time is the problem. You have to accept that the ratio in your premix will change once the flavourings have been added. If you're relaxed about that, and not pedantic, then you're on a good path. Personally I don't care whether my home made has 10, 11 or 12mg/ml of nicotine because I don't notice the difference. You will self-regulate your blood levels when vaping anyway.
The key is to work out what ratio you want your final mixed juice to be, roughly, and make up your premix accordingly. Allow for say 10-20% flavouring of PG based concentrate and you'll know what you're vaping. If you want to end up with a 50/50 mix after the 10% of PG flavouring has gone in then you make a premade base which is higher in VG. The maths is pretty simple.
If your premixed base is 50/50 with 6mg/ml nic and you want to add 10% - or 1ml of PG based flavouring - your final 10ml mix will be 45VG 55PG:-
9mls of Base = 4.5mls VG, 4.5mls PG
1ml flavour = 1ml PG
You will have diluted the nic strength from 6mg in the premade base to (9 x 6 / 10 =) 5.4mg/ml
It might well be that the range is fine, would you notice a difference between 50/50 and 45/55, or feel the difference between 6mg and 5.4mg? Does it offend the pedant inside you?
Well, you have the option to adjust the numbers ... mix up your base at 55VG 45PG with nic strength of 6.7%:-
9mls of Base = 5mls VG 4mls PG
1ml flavour = 1ml PG
Now your final juice is 50/50 at 6mg/ml.
That's all fine if all your mixes use 10% flavour, add more or less flavour according to the recipe and the ratios change again. Now you either decide to live with the minor fluctuations or mix from scratch every time. Personally, I'm happy to vape within ranges of VG/PG and nic strength because I think the most important thing to get right is the % of flavouring going into the base.
For my personal use I mix a base using 45mls of 72mg nic in PG and 205mls of VG - this gives me 250mls of premixed base at 82VG 18PG and 13mg/ml
(I want a high VG ratio in my juice but I buy my nic in PG because it stores longer in a freezer. Most flavourings come in 100PG but some are 50/50)
After adding 10% PG flavour the final juice will be 70/30 12mg
After adding 20% PG flavour the final juice will be 65/35 11mg
I don't notice any difference between vaping a 70/30 and a 65/35, I reckon it would be impossible to discern the difference between 12mg and 11mg. So that's more than good enough for me.
