Tony, I never worry about inertia, except my own.
Peter, I think the total inertia is the sum of the inertia of the tonearm in front of the pivot plus that of the stub and counterweight and anything else behind the pivot. I think what you are referring to is the counter-intuitive fact that the moi of the rear part of the tonearm is related to the square of the distance from the pivot to the center of mass of the counterweight and to only the first power of the mass of the counterweight. Thus, a heavier counterweight that can be moved closer to the pivot will result in lower moi.
Peter, I think the total inertia is the sum of the inertia of the tonearm in front of the pivot plus that of the stub and counterweight and anything else behind the pivot. I think what you are referring to is the counter-intuitive fact that the moi of the rear part of the tonearm is related to the square of the distance from the pivot to the center of mass of the counterweight and to only the first power of the mass of the counterweight. Thus, a heavier counterweight that can be moved closer to the pivot will result in lower moi.