Off the top of my head I can tell you 9 wraps each of 26g will get you 0.5 ohm. So using 25g you might want to try 10 wraps each, or just do the 9 wraps and have a little lower resistance. On a regulated mod resistance isn't so important as long as your mod can safely fire it, and your batteries are safe for the power level you're using.
I have a dual twisted 28g build in one of my tanks. 9 wraps each on 2.5mm for 0.55 ohm.
The Billow v2 deck is on the smaller side, too. The Indulgence MT/Mutank and Geekvape Griffin have good sized decks.