I found the Urbanist article, which gives nice visualisations.
I also found
this, which gives 500 dwellings per
hectare as a 15 storey building. That's about 200/acre, so your 648 would be a 50 storey building, which is not unheard of now.
I'll redo it with your adjustments though. Do you reckon 2.5 ppl per dwelling is reasonable then?