Welcome!
The mod itself is getting pretty good reviews, but I don't hear a lot good about the tank. I have never used either one. It is true, and you have seen, that bigger is not always better. If the SubVod is working good for you, I would suggest picking up a SubTank Mini or a SubTank Plus to sit on top of the Fuchai. They use the same coils as the tank on the SubVod, and will give you a bit more tank capacity.
As for the settings, you just have to kind of set it a bit low and work your way up. It takes some fiddling you will find a sweet spot.
The juice suggestion is correct. If you were using a Kanger Protank or somthing with lower power and airflow I would suggest 18mg nicotine, for the SubTank Nano, you are right on the mark at 9mg. The other tank 9 may be too much. As you increase the wattage, and airflow you need to decrease your nicotine level. As you are fining out, there is an overall sweet spot where you are getting good airflow, satisfying vapor, and good flavor. Normally with factory coils you as you step out of your sweet spot you start loosing things, usually flavor as you go up in power.
Since you are new around here I will give you the mandatory suggestion to buy backups, and backups for backups. It will save you!
Welcome to Vapor Talk and feel free to ask questions, it is the best, safest, and cheapest way to get this stuff figured out.