A more realistic estimate of the total number of addressable things should take into account that for anything to be addressable, its address should be stored somewhere at least once.

If it takes at least Npb particles to store one bit of information, then the number of addressable things would decrease with the number of bits of the address.

So let's call Nthg the number of addressable things, and assume the average number of bits per address grows with Nb = f(Ntng).

Then the maximum number of addressable things is the number that satisfies Nthg = Np/(Npb*f(Ntng)), where Np is the total number of particles.