Well, the LD50 (the dose required to kill half of subjects who inject the substance) for SN is set at 71mg/kg in humans, or 0.071g/kg. That would be a 7.1g dosage for a 100kg person for reference. Realistically, nobody is looking to leave their suicide up to 50/50 odds and accounting for differences in body reaction I think the currently established dosages are reasonable.
Vomiting based failure is just a fact of the method, honestly i'd be more inclined to look at the supporting drugs people are using in their SN protocol and if they're adequately testing their reactions together before their attempt. Complete anecdote but initial doses of propranolol make me nauseous, if i didn't know that and added it into my plan it would be a major unconsidered point of failure.