Not sure there's an "average" weight gain, but given your numbers ... you're set dude! She's primed to go. My girls are all over the map, the first is about to drop eggs in three weeks and her weight has hovered around 1800gms ... a couple others are 3000+. But in my experience, weight ranges don't matter. Just know, the smaller the girl, same with the clutch. But its all about healthy eggs at this point. Sounds like you're doing the right things ... good luck.