I don't have an IPV4, only use mech mods. But if I was going to build it the first time, it would either be dual 22g coils 2mm ID or dual 24g coils 2mm ID. Either with 6-wraps per coil. ohms out to about .14 or .21, respectively. You probably don't need to do that with your regulated mod as you can just crank up the volts, but you asked me what ohms I build to.
EDIT: sorry, I just notice you are talking about an RTA. I was talking about RDAs. How much room do you have? If you can fit the 24, do that build. Otherwise, 26g might be a better option... probably will end up above .3 ohm if you do it the same way.
You have a regulated mod with protection. So nothing I mentioned is unsafe.