After reading around the forum about this, I have to say I don't understand why such a high dose of benzos is recommended. I think the idea is that you go unconscious as quickly as possible...but there's a potential problem: if you go unconscious "early" (before the SN would make you go unconscious) then there's a possibility you might vomit and not realize it because you're unconscious (from the benzos). Then you wouldn't be able to take a second or third drink, and then your attempt possibly fails.
Because the SN would make you go unconscious within 15-20 minutes, I personally plan to use benzos in a much smaller dose about an hour before SN just to keep me very relaxed but still alert enough to take another drink or two if needed. My understanding is that if the SN makes you go unconscious (and you've followed the protocol for fasting, limiting water, SN dose, etc), then you've made it to a point where it's very unlikely you'll survive unless someone finds you early enough.