(Began as a reply in another part of the ECF forums; slightly modified here)
Mixing is good. My own e-juice calculator is just doing the calculations in my head (or with a standard calculator) with a few notes on paper. Because of this, I strive to end up with juice formulas as simple as I can make them. My current "formula" is two parts of a weak nic base mixed with one part of a stronger nic base. It's hard to get much more simple than that.
My own e-juice is very strong (65 mg/ml in 68/32 PG/VG), so yours might be something more like one part nic base, 4 parts PG and 5 parts VG. I find that the actual PG/VG ratio is really not very critical, so if you have found that you like mostly-VG juices, then probably anything between 60% and 80% VG will work for you. This lets you choose an easy-to-mix formula that ends up anywhere in that range.
Nicotine content is likely a little more critical, especially if you want fairly low amounts of nicotine. Still, if you are aiming for 12 mg/ml (for example) you will probably be able to use anything from 10 to 14 mg/ml (roughly plus or minus 15%). Again, this lets you choose a mixing formula that is less complicated than some "exact" formula that calls for mixing 13.243574 ml of PG plus 47.9846584 ml of VG and 4.73642 ml of nic base.
If you like e-juice that is mostly one or the other of PG or VG, consider getting your nic-base in the carrier that you want the least of in your final formula. For instance, if you want to end up with 10 mg/ml nicotine and 90% VG, get your nic base as 100 mg/ml nicotine in 100% PG. Then you would just have to mix one part of your nic base with nine parts VG, and voila! there is your e-juice with an easy to mix formula. You could use the same method to end up with 20 mg/ml nicotine and 80% VG by mixing at one to four.
My point is: it is MUCH more important to end up with a simple formula than it is to hit some exact PG/VG ratio and nic strength. For one thing it will make it MUCH less likely that you will make a mistake while mixing that means you have to toss a batch of juice, or (worse) maybe spend a few weeks wondering why vaping is not satisfying your nic craving (or maybe making you slightly nic-sick for those weeks).
As far as mixing by weight vs mixing by volume: PG, VG, and nicotine are all close enough to 1 gram/ml (SG of near 1.0) that it makes little difference, especially if you take my suggestion that exact ratios are fairly unimportant. What you are aiming for is a simple to mix formula that works for you. What do you care about exact numbers as long as you are satisfied by your result? And that you can mix it week after week, month after month, without making mistakes or sweating over amounts (or weights) too small to really matter.
I'm torn, because I agree with a lot of what you say (in ways that go beyond mixing juice,) but kind of disagree about some of it when it comes to mixing, at least in the general case (not disagreeing about how well it works in your specific case.)
First off, if you're mixing purely for yourself, by trial-and-error, with all flavors in PG and a consistent PG/VG ratio replicability will come down to the precision of your instruments, and the specific gravity of PG vs VG won't matter (the differences in specific gravity of different PG-based flavorings are inconsequential given the amounts used, so I won't mention them other than to discount them as a factor.)
That said, PG has a specific gravity close to 1.0, and VG has a specific gravity around 1.25. That's a not inconsequential difference, IMHO. Differences on that order might not usually make a recipe I love a recipe I hate, but they might well make a recipe I love one that I don't love so much. And because of certain threshold effects with flavorings that kind of discrepancy can occasionally mean the difference between love and hate.
And when we start to communicate about mixing we need a common language to communicate with, and for better or for worse we've decided on weight as that language, even when we mix by volume (not sure that was the best choice, in the face of varying PG/VG ratios. Measuring by volume might better reflect subjective experience there, but it's a very complicated question. I kind of doubt it matters much when going from 30/70 to, say, 50/50.)
I disagree that nic concentration is the more important factor here. You won't probably notice a 30% difference in nic- you'll just vape more or less and get the same amount of nic. Nicotine addicts are remarkably adept at titrating their nicotine intake. You _will_ notice ~30% differences in flavors though. What's tasty at 3% is often gross at 4%.
And finally, while I agree that consequential processes that make error likely should be avoided, mixing juice is a lot more like making biscuits than it is like running a nuclear power plant. I've mixed a lot of juice, I have a routine around it, and I can't recall the last time I really screwed it up. And if I did screw it up a bit, next time I mix, would there be any serious consequences? Nah- I'd just throw the 75 cents worth of ingredients down the sink.
