When I first started vaping, the standard mixes they had for tasting in the vape shop were 12mg strength. I wasn't able to inhale them, and it made me cough. They recommended I start with 6mg. Even though I smoked ultra lights, I was smoking 2 packs a day, and I thought 6mg was pretty low for me, but at least I made the transition and learned to inhale on that strength. (I do know this form of nicotine isn't absorbed via the lungs, only the mouth and nasal membranes, but it didn't feel like smoking to me if I didn't inhale.) I used 6mg for a week or so, then moved up to 12, which I decided was just a bit too strong. I've been making my own e-liquid for over half a year now, and I usually make it about 8-10mg. Still even use 6mg if I really want to just vape a lot some days. The higher strengths tend to give me acid indigestion.
You said you bought 15mg and then 6mg. If by chance both are the same flavor, you could combine equal parts of each one, and the result would be about 11.5mg. You could even further play with it from there, combining the 11.5mg with the 6mg to come up with a bit less than 9mg. Everyone is different. Just experiment some, and you'll find what's right for you.
And many congratulations to you! I also smoked for over 40 years, more like over 45 years. It's wonderful not to do that any longer. My one year anniversary is coming up on July 2nd. You can do this!