I have ordered a Kanthal GA 30 on ebay, will give it a shot. From what I have seen on Youtube, the 50 watts is to dry?? What is a decent mod for dripping?
You can get RDAs that are designed for anywhere between 7-100+ watts. IMO the Mad Hatter is designed for the higher end of that range, but I don't have one, I only had a look at it online and that's just my opinion from looking at a picture.
If you try to set this up for 20 watt use with 30 gauge wire you have 2 options - a single coil that I would aim for 1.7 ohms with, or dual coils at 0.6 ohms total. The single coil build will be the warmer vape at 20 watts, but you'll have to be careful not to go lower than 1.7 ohms or IMO you'll start burning juice at 20 watts and you'll have to lower the power.
That RDA has two large air flow channels from under the coils that you can close down (and you will want to close them down at 20 watts), but if you use a single coil I think you'll want to close off the other one completely or you'll lose a lot of vapor and flavor.
Best of luck with it.