The way Maj H designed artillery effectiveness may not be precise, but it is generally right (that is, about as good as can be expected -- if not better -- for a desktop computer game.) More importantly, it gets you to react to artillery in a doctrinally correct manner. (e.g., don't occupy terrain that is so obviously good that the enemy will prep it, don't stay anywhere after receiving effective fire.) No tank is designed to take a direct hit from a 152/155mm of ANYTHING. Especially since it is hitting the TOP (weak armor) of a tank. ICM, which disperses many, many small HEAT charges means that (in many cases) the arty shell can have good effect against armor over a larger area. It may not totally destroy the tank, but it can be good enough to put the tank out of action for the period covered in the scenario.
LTC Chris Mayer