I'll chime in a little help here... As everyone said earlier, the simple solution is to use your Nautilus Mini on the new mod, switch it to Wattage-mode, and use somewhere between 9-12W, which is probably what you were using on the old iStick 30W? This is the simplest solution and you'll be happily vaping the same as you were before the iStick 30 died. There's no need to sub-ohm or use TC-mode unless you want to, which leads me to this...
When you drop resistance of your coil... you also have to lower your Nicotine strength by roughly the same ratio. For example, if you were using a 1.8 Ohm coil and 8mg Nic strength, and want to use a 0.8-1.0 ohm coil... you'd want to drop your Nicotine down to 4 or 5mg. Dropping from 1.8 Ohms down to 0.3 Ohms is a major drop... (over 80%) so you'd want to drop your Nicotine level down to 1 or 2mg to prevent harshness and to maintain your "normal" Nicotine dose-level (Nicotine absorbs faster into the body as it gets hotter). You'll also want to increase your VG ratio to 30/70 PG/VG as a good ballpark for sub-ohm vaping.