Starting out at 385 lbs means there's still real distance to cover, and you'll almost certainly land on a full dose eventually, I'd argue 12mg ends up being the right target for the best combination of weight loss, maintenance, and long-term health outcomes.
20 lbs in four weeks moves fast, was your weight holding steady before starting reta? Given you mentioned dropping 55 lbs before that, if things were stable even briefly beforehand, a chunk of that recent 20, maybe 10 lbs, could just be water rather than actual fat.
Expect the coming weeks to show a slower, more realistic pace, sustaining over 2kg weekly for any real stretch of time is basically impossible unless someone's carrying a lot of extra weight, eating very little, and staying highly active.
At this rate there's no real rush to bump the dose further unless hunger becomes an issue, though long term I'd lean toward aiming higher rather than settling low. Most people around here will tell you to run the smallest dose that works, and I don't think that advice holds when someone's starting weight is this extreme, provided the side effects stay tolerable.