I used Mistics for a month or so. The original liquid in their cartos is 100% PG, I believe. You could theoretically use any PG/VG ratio. However the higher the VG the harder it could be on any carto. Then you have to factor in the quality of the specific cartos, also the quality of the VG in the
juice.
I never tried to refill the cartos with my Mistic kit, because that was back when I didn't know anything about
vaping lol. I didn't even know that you could buy liquid to refill with at that time. I just used them until they were dry and then threw them out...which sounds absurd to me now.
You could probably use a high VG content, but it may shorten the life of the cartomizers. But then again I had one VG juice that was quite merciful to my cartos and let them live quite prosperous lives.