Howdy,
Welcome to ECF
As for the building of a coil, a lot goes in to play and calculators can give you some of the answers you seek but don't really answer questions in a sense.
I must warn though, as mentioned please do be careful when attempting to play with RDA's or RTA's as you can exceed the amperage limitations of your batteries without even knowing it. That being said staying around the .4 to .6 ohm will not stress to many new batteries on the market today.
It boils down to one truth, Ohms per inch/foot of wire. The lower gauge will have a lower resistance resulting in a lower ohms per foot of wire however...driving that system will increase amperage requirements. The size of your coil wraps increasing will increase the amount of wire needed per wrap thusly increasing the ohms (more wire used concept).
Also keep in mind that your wattage (heat) will increase with the drop in Ohms, typically speaking up until a point all wire will reach a maximum temperature eventually however the lower the resistance just means how fast it gets up to that maximum temperature (lower ohms means faster heat up). Finding that right mixture is pretty easy on a VW/VV mod however on a mechanical mod it requires a bit of tweaking to get the ideal vape, this is where your calculator app's get used a lot.
Hope this helps out a bit, feel free to hit me up in PM should you need any questions answered or advice, I've been at this since long before RDA's and RTA's were even a concept on the drawing board.