Disclaimer: I am by NO means an expert on this topic and everything I say is anecdotal and/or gleaned from sources available to everyone here.
That said, it seems to be that if someone fails to ctb with SN (especially a dose like 25g) its for one of two reasons. One is that they vomited the solution before they could absorb a fatal dose of it, and didn't redose. Second is that they're "rescued", whether that's because they panicked and called EMS or they were discovered and someone else did.
From what I understand, the LD-50 for people is about 71mg/kg. This is just shy of 5.5g for you. However, keep in mind the LD-50 is only fatal *half* of the time at these doses. So if you really want to ctb, you'd want to exceed it, and the more you can exceed it by, the better for your purposes. The reason (I believe) that 25g is recommended and backup doses are too, is a matter of absorption. It takes into account that many people vomit, and its impossible to know how much you've absorbed when you do. Case of "go big to go home" if you ask me ='D