@HopefulSleep About the grams, it depends. There are different variations out there because everyone is different. Male and female bodies digest stuff slightly different. Generally everyone has a unique genetic makeup that affects how much they would technically need. The problem with taking too much is that you might not be able to keep it down. If you vomit too much your death will before painful as it will be slower. Technically speaking, people have died from as little as 2-3 grams before, while some people claim to have taken 15 grams (not vomited) and lived.
If you look at the data most unsuccessful attempts were caused by either lack of preparation (no meto, having a bad reaction to meto, taking too little sn) or being found (within those 4 hours). That does not however say anything about how the other people died it just means they died. That's why the time can range from 40 minutes to 4 hours.
The fasting also depends on your body. My metabolism is really slow so I aim to fast a little longer than the recommended 8 hours just in case.
Goal is to loose consciousness before you die, so you don't feel as much. You can try to achieve that by staying calm and in your bed after ingestion and staying awake as long as possible and/or taking enough benzos before ingestion.
For me personally, benzos probably won't do as much as they might do for you as I have really bad anxiety and just calming me down from a bad panic attack takes at least 4mg aplrazolam. It won't knock me out though. I've never ingested any thing, be it sleeping pills, medication or drugs that instantly knock me out. Stuff has made me a little sleepy, yes but that's about it (unless I take a ridiculous amount).
You know your body best, you know your general tolerances. If you often need a lot of a specific substance to get high, take more SN. If you tend to get nauseous quicky, I'd suggest taking a little less to reduce the risk of vomiting.
The most important thing is so stay calm. Anxiety can feel like a painful death by itself sometimes. The issue with SN is that you can feel your body shutting down before you pass out, your breathing will become heavier and your heart will beat faster. If that causes you anxiety you are going to have a hard time but you could say the same about any method, besides maybe N.
I know of two cases where there was a long, painful death struggle
You're ingesting a shit ton of poison, of course it's not gonna be entirely painless.