Yeah I agree with your math (well I haven’t checked it but it sounds about what I’d expect). But 7% isn’t a big amount within the other approximations going on here. The point is that it doesn’t double using helium even though the density is double because the relevant change is the difference between air and the other gas - which as you say is only 7%. Like if you could somehow make a weightless container that had a perfect vacuum in it, you’d only gain another 7%.