I smoked for 35+ years. Started @ about a PAD. It seemed that every failed attempt at quitting bumped my smoking habit up. In the last years of smoking, I was going through 2 bags of rez smokes, the strong ones. I've been vaping 21 months now. I started with 24mg juice and stayed with that for 11 months. Dropped to 18mg for about 3 months, then down to 12mg, That seems to be my sweet stop spot.
Before I was able to kick the smokes out of my life for good, I vaped & smoked for 6 weeks.
My advise for any new vaper -Don't stress yourself, if you need to do both for a smooth transition, do it. Every analog you don't have is a huge step forward. Just enjoy the new venture.