I've been doing it in various ratios... It really depends on my mood or how my taste buds are doing... I have one reo with unflavored all the time and when I want to try some flavor I saturate the wick, then keep squonking, till I feel the need to add more flavor. It really makes my taste buds pick up every note of the flavor. In my other reo, I've been doing anywhere from 25%-75% unflavored straight VG, depending on how I'm feeling... (in that reo, its been wakonda/virus)... I sometimes want more flavor or more vapor... Mostly because I've been using an Odin and doing lung hits with the reo's...In my magma, I will run straight flavored, and then I might top it off with some unflavored if I start to notice its too strong, and then I can start to taste it again (with too many flavors too name...GCD right now...)... It's really working out for me, because I seem to get flavor shot pretty quick, and this way allows me to consistently taste em.
I'd suggest you try a 6mg unflavored bottle in the reo, then drip some 12mg on top. OR just mix it in the reo bottle to get a lower nic overall.. Clark's suggestion on getting NZ is probably what you should do as well... I have done that for all my 12 mg's, except BG and Gravity... Which, Im planning on mixing with straight 0mg VG to see where they land... that reminds me... I need to try that out.
Also, note, you might need to adjust the coil/wick to compensate for thick VG... squonking takes a helluva lot longer (sometimes 10 seconds if below halfway for my odin with large wells)...I just do micro coils with KGD... but at first, I was using too much wick for the VG... at least I think.