What PG/VG ratio is your juice? When I have higher PG, I don't get quite the volume of vapor as I do with higher VG.
I use around 6mg in my lower-ohmmed toppers. Around 9mg at higher ohms. It's all about what you can handle. I read that some folks hit 18-24mg at low ohms and it doesn't bother them. Everyone is different.
The Subtanks are closer to a "dripping" experience than more conventional tanks. Dripping, by nature, delivers nicotine in a higher concentration/puff than a conventional tank, so you will see drippers and sub-ohmmers who will bump their nic levels down. You could pick up some USP Grade VG from Walmart or some pharmacy section and cut a bit of your current juice with it. You'll reduce both the nic level as well as the flavor, though. The extra VG should improve the density of the cloud, and the lower nic should be a little easier for you to handle.
You're likely taking more of a direct-lung hit, which is a little more rugged to handle unless you're used to it.
Good Luck
ETA - sorry, you replied to another poster before I could finish babbling. I find that 50/50 doesn't give me much cloud on the Subtank - I've had to up my VG when I use those tanks.
