Do you think if I use the largest airflow insert and open the airflow on the bottom of the tank fully I can take a direct to lung drag for the salt nic? This way I'll have the option, I can tighten up the airflow on the bottom (while the largest insert is in still) to get a mtl hit if I want to?
These days I vape everything direct to lung (DL), and my tanks and RDA's have restricted direct lung air flows (RDL). For me that means the air flow is between a loose MTL to a little less than what people consider just into full DL. Ultimately, you can vape any air flow DL, but if the atomizer is designed for true MTL it will take a
long DL drag to get it done
Yes, if you install a big air flow hole insert you'll certainly be able to reduce air flow of the tank with the air flow ring, but the hole size under the coil also affects the vape regardless of how fast you draw. If you adjust only the air flow ring to give you a full MTL draw in this scenario, my bet is you'll also want a smaller air flow insert hole so it gives you a better MTL vape.
IMO at this point you're overthinking this a bit EIHYPI. Keep in mind that you can easily change the airflow insert, the tank air flow ring setting, the coil and the juice as you see fit. This means you don't have to get it right the first time, and my bet is no matter how much you try to think through every variable you are very unlikely to get it perfect the first time. If you make a good guess at how to set up this tank the first time you will have a good chance at getting a reasonable vape right off the bat. Then it's all about tweaking things from your experience with the setup(s) you've tried in mind to get it to where you want it. One thing I would suggest is that you vape one setup for a few days minimum making only minor changes so you give yourself time to really get to know a setup before you move to the next one.