I'm also wondering about that threshold. Right now I'm only using three quarters of a milligram twice weekly since I only had fifteen pounds total to drop, mostly the visceral kind.
There's been plenty of discussion on whether 4mg or higher is really where the advantages over similar meds show up. From what I gather the glucagon part doesn't kick in until you reach 4mg or more each week. I worked my way from half a milligram up to 4mg across a couple months and have held there for three weeks now without noticing any shift beyond the steady progress I've already been getting.