I didn't read this anywhere, it's kind of like the new Nic shots with the juice that's specially made for it, the vendors wouldn't be advertising a 70/30 3mg mix as exact.
Because as you mentioned adding concentrates would screw the Mix and it wouldn't be 70/30 anymore, loads of places that sell concentrates and premix recommend that if you use their own one shots they recommend 15% therefore the pre mixes must be set up for this or they wouldn't legally be allowed to advertise these products on their sites as a 70/30 Mix.
The Mix part being the key word here, now a 70/30 solution would be a different story.
Plus the fact that PG cost slightly more than VG, only by a small amount, although still enough to bring costs up when making massive batches.
I have been mixing and selling my own juice for just under a year and have learned a lot in that time and if it were me selling a 70/30 Mix it's kind of a given that I would leave room for the average amount of flavourings at 15% which I'm sure you are aware are mainly PG based, so it makes sense to leave out 15% of PG, this way the Mix would be 70/30 as mentioned above if it was called a solution then it would or maybe 70/30 as it comes, although this means that, as you mentioned as soon as the concentrates were added the PG levels would be different.
So why advertise a product as a certain Mix when it's not?
Take a look at how certain companies are getting around the TPD by selling 60ml bottles of the same juice as they did before mixed at 0 nic which is legal.
Then supplying a nic shot in a 10ml bottle which when added to the 60ml bottles, which don't contain 60ml of juice, as for one the 10ml of nic shot wouldn't fit and once again the ratios would be off. So the premix they use has taken into account, not the concentrates as above but the Nic shot so if the nic shot is pg, the bottle would contain 10ml less PG. So that when the nic shot is added the exact ratio is achieved.
It has to be the same with 70/30 premix, as the name implies?
Some big name Juice added to oneshots nic are a good example of this, have a look about, I know this relates to getting around the TPD and selling 60Ml of juice when they can only sell 10ml with nic legally, although their 60ml bottles only contain 50ml with 0 nic which is legal, all ready for the 10ml of nic to be added. It has to be the same for premix?
It seems kind of obvious to me, although everyone is entitled to their opinion and hey that's why we have free speech
heres an example of the new get around, I copied and pasted this from a vendors site for a well known juice, I'm not gonna put where as I'm not advertising for them on a DIY forum as they don't sell concentrates:
Note nic brand of Oneshot has been changed as in, it's not called Oneshot it's called something else, Oneshot just seems to be the new term most vapers are using for their nic shots ...
Details of example from a Juice retailers site
50ML of juice in 60ml bottle ratio of 50/50
oneshot Ready = Ratio ready bottle, designed for use with Oneshot nic, not suitable to vape without pouring in a Oneshot nic bottle to reach desired Nicotine Strength, Oneshot Nic sold separately. 3MG is the maximum Nicotine level that can be reached. 50ml bottle; 60ml after Oneshot Nic is used.
Oneshot Nic Ratio Guide:
30ml requires 1/2 bottle of Oneshot
60ml requires 1 full bottle of Oneshot
120ml requires 2 full bottles of Oneshot
The above will result in a 3mg formulation
Now a lot of these juices are marked as 50/50 Oneshot ready, yet the Oneshot that comes with them are advertised as 70/30.
Again how can they market a specific ratio if it will change when the Oneshot is added, surely they must have done the calculations?
Just my thoughts, personally I do not use premix , just helping out a fellow DIYer
I welcome your thoughts and anyone else's, I do have a few vendors phone numbers and tried to get in touch while writing this post, although none were available, I will however see if I can get hold of someone who runs a shop and sells premix to tell me if I am wrong or right.
Although I am right 99.9% of the time lol
Look forward to hearing from you
All the best