Looks like I'm just going to stick around the 3.35-3.4 volt mark. Seems to be working for me at the moment.
If you look at this chart, your level is just fine for safe and normal operation.
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Some juices like more, some less. I have a couple I'm still looking for the sweet spot with!
If you want good flavor and vapor, try a 50/50 blend, it's what I like - you'll still get a lot more vapor at lower levels than the mix you have.