15g is a high dosage, and is *very* likely to be fatal (unless your bodyweight is very high), however a slightly higher amount might be advisable to be "completely sure".
100mg per kg is considered a potentially lethal dose.
So if you weighed 70kg, then 7g is potentially lethal and 14g would be very likely to be lethal.
To be even more certain we tend to suggest using higher amounts.
As @Jumper Geo wrote above, I have a table which some people find useful :
https://sanctioned-suicide.net/threads/sn-water-amount-lookup-table-v2.32241/
Some other general notes I have :
RELIABILITY
If you follow the protocol (Stan's guide) closely, and have a certain amount of determination to undergo some discomfort without calling an ambulance, and provided you take precautions to not be found, then the likelihood of the method failing is very low.
Some members have vomited, but still been successful. In the cases where people vomited it doesn't appear they drank any more SN, yet they still appeared to succeed, although the recommendation is to have a couple of extra glasses ready, and to drink more in the event of vomiting. For cases where the method has failed (mostly due to the reasons stated), the recovery seems to be good, and there don't seem to be many reports of serious lasting damage.
PEACEFULNESS
I've read various accounts of people taking SN, from various sources.
The overall impression I get is that it is fairly peaceful.
I believe there will likely be some discomfort, but I don't believe the discomfort level is particularly high.
I have heard it described as being like a "bad hangover".
I believe people usually go unconscious after about 20 minutes.
Note : I have stated what I believe to be true and the impression I have obtained, based on things I have read and things that have been posted by others. I cannot give guarantees.