Hi Mikenet,
I would just like to say that when I began DIY 4 months ago and entered the world of "attys", I found a thread here talking about the burnt taste. The consensus was it is common for new attys to taste bad for several hits (several days in some peoples experience) and that it is probably whatever lubricant is placed in them at the factory during manufacturing. The common solution seems to be to mix a batch of "plain" juice with either just vg/dw or vg/nic/pg depending on your preference. With any new atty you would then vape a few hits of "plain" through it to remove whatever factory swill is present before adding your flavored juice. I would also recommend vaping a drop or two of plain in between flavors if you have a day when you are testing a bunch of different juices.
Secondly, I suspect from personal experience that all the shaking and warming in the world (ok maybe that's extreme) is not enough to effectively distribute flavor in one day in a 40+ % vg juice. Set that one aside for a week and try it again. Meanwhile you can move on to your other theories.
Thirdly - I implore you, beg you and bow low to say pretty pleeeease - Mix some smaller batches of nic at lower levels before putting it into your recipe. I use batches plural because you were not sure how much vg you wanted in the end product and there is anecdotal consensus that it "holds up better" in the pg base. Please at least make a 20 ml bottle of 50% vg and 50% your nic. You are not opposed to doing math to find your perfect juice - that is clear from this thread

It's just that the stuff is so dangerous at that 10% level and it is soooo easy to just make a tiny mistake as you are already finding. Please at least cut your risk in half with the nic. Once you find your perfect blend you can cut more at a more useful ratio if there is one. I "heart" all my fellow vapers and I advise out of respect and care, nothing else.
You appear to be well on your way - enjoy and vape happy!
Thanks for the tips, Ive done a couple things now to confirm my mixing procedure was done correctly. There was some good news and bad.
The good news is that I did a nicotine test on the first 11ml batch that had a target of 26mg per ml of nicotine. It tested as 24.5 MG per ML. Considering the nicotine test has a 10% margin for error, I'm very happy with this result.
The bad news is that the second 6ml batch tasted just like the first one, so I didn't error after all but I'm still not sure why the flavor is so weak.
Since both these batches were identical, I combined them into one bottle. So now I have over 10ml of weak tasting liquid. After this I decided to up the flavor by 4%, bringing It up to 14% of the total volume. I also added the recommended 1 drop of MTS Vape Wizard in an attempt to boost the flavor.
Again I went through the whole routine of heating, and shaking, then repeating. I waited for a few hours and vaped it after shaking it again. It almost taste like I did nothing, it's still very weak.
So that's the bad news because I really don't like waiting. Ive been reading up on steeping a little bit and I'm really taken back that I might have to wait a week like your saying, I'll do it if it's necessary though.
About the nicotine base, I really hear what your saying about making a lower nic base to work with from the 100mg nic I have. It makes perfect sense for safety purposes, but I decided not to primarily because I have no idea what pg/vg ratio Im going to be set on, and unfortunately I didn't buy enough bottles for several batches. I did fill one dropper bottle with PG for atty dripping in between flavors though.

Still have to give that one a try
Another reason i didn't make diluted bases is because there is now a wild card with the nicotine level I may end up liking too. I thought all the premixed I was buying was either 18 or 24 mg but I tested 3 bottles of premixed and they all tested below 8mg. It might be that some of the premixed has ingredients that are skewing the test results, but my premixed tested accurately as did the nicotine base.
There is one last reason I decided not to make a diluted nic base. In my mind it increases the amount of times I'll have to measure, and since syringes and pipettes aren't 100% accurate, the more I measure the less accurate my final mix will be.
I was thinking measuring once from every bottle would give me mixes that are somewhat closer to their target ratios.
However, I would like to know if their is any disadvantage to directly extracting the 100mg base, other than the safety concern. I am wearing gloves and being very careful. Would it take the mix longer to steep if I add the 100mg directly? Or affect the flavor in any way?
Thanks