For the game to be balanced the player that spends more mass (wisely -- not just on AA, for example) wins. So if your opponent spends x amount of mass, and the game is balanced, you should have to counter it with x amount of mass.
T3 arty costs 70k mass. A T3 shield costs 2.4k mass. So you should need 70/2.4 = 30 T3 shields to counter a T3 arty.
Unless you want to unbalance the game?
Honestly, by the time your opponent has spent 70k mass, if you HAVEN'T build something comparable (an arty of your own, an experimental (or 2 -- a fatty costs only 28k), etc), you SHOULD lose.