Haha, I used to do that exact build when all I had was a tube mod. Solid choice. I also still own one of those Sigelei 150 TC's. One of their last really good mods to date.
Since you're using a Sig 150, you don't have to worry about resistance. Because of how the mod gets and delivers power, the draw on the battery depends on the wattage rather than the build. Fresh batteries (8.4v) give you 150w for 19A, regardless of build resistance. But as they deplete, it'll exceed the 20A limit of your batteries, needing to pull 26A by the very end of their life cycles (6.4v.) 125w will take you to 21A just before the cutoff kicks in.
So we can assume the limits for being completely in the clear are 125w (current limit for your batteries) and .1 ohms (mod's resistance limit.) Keep those in mind and you can do pretty much any build you want with that 20g and be completely safe.
It's tough to balance 20g optimally without building dangerously/unusably low. In order to get it to heat-up quickly, you will need a lot of power. Unfortunately, there is a point where your coils wind up heating up much faster than the heat is dissipating. And on top of that, the mass and amount of surface-area carry very heavy wicking demands. Lots of heat moving a ton of juice.
This leaves you with a catch-22... ...the amount of power needed to get the coils going leads to an accumulation of heat that makes them unvapable after just a few consecutive drags. This form of heat-retention also affects wicking, as the sheer amount of heat moving around overcomes wicking and airflow's cooling capacity. The wicking drops off very quickly at this point... ...usually towards the end of the first drag. Once the temp crosses that point, it turns over and shoots way up.
I can only guess as to whether or not the builds below will work. The lowest I've gone is 21g kanthal and that's what I'm basing these ideas on. It's worth mentioning that I never use the 21g for the reasons cited above...
Personally, I would try to build as low as possible with the 20g... ...just to cut down on the mass and make the heat more manageable.
You can try to get a .1 by doing a 5-wrap, 3.5mm dual-coil build. This will still have plenty of surface area, but hopefully won't need as much power to heat-up and should still wick okay. These would still be pretty massive, but they should work.
125w should do it for that build. You may need just a little more. If you want to go past 125w with those batteries, go ahead, for now. In the meantime, get going on finding an alternative and don't run the batteries down to "low battery" cutoff.
Honestly, a single might be more managable. You could do 6-8 wraps @ 3.125mm-3.5mm. Maybe clapton it and take a wrap or two off.
One last thing. Shorts in a ~.1 build will put it under .1 and trigger the "short" cutoff in the Sig 150 TC. The way around this is to either torch the coils beforehand, or loosen one when it dips so it'll read higher and let you pulse the shorts out. Once the shorts are out, it'll fire just fine, but you have to trick the mod into letting you set it up.
