Your resistance is NOT going to be around 0.3 if you use dual coils, at least at that resistance. Dual coils HALVE your resistance, so with that build you will be vaping at 0.15. I would not recommend doing that on a single battery mod, so your choices may be to get a different RTA for that mod (probably a single coil RTA or RDA would work best) and build a coil in a higher range. Absolutely NOTHING wrong with starting out higher (0.5 is a good, safe place) and seeing how you like the vape, with a single coil. You can then start to experiment with slightly lower builds, but I wouldn't recommend starting there or lower trying to get used to your new equipment. Or you will need a dual battery mech to give you the safety and headroom you need for what would be a 0.15 build (if you are lucky, and don't make any errors during the building process.) Also, most ohms readers are not supremely accurate, so building in safety is always a good idea.
Best of luck, I would save your single battery mechanical mod and pair it with a more well suited, single coil RDA or RTA.
Best of luck,
Anna