If you are in an accurate arty (object 261 or all the french arty) then what you aim for really depends on how the tank is oriented. If he's facing towards you, his back end is almost impossible to hit without risking an altogether miss (since the turret is in the way). At that point aim for the top of the turret. In other instances, try to aim for the top of the back end of the tank.?? about arty:
Do I aim for the top of the turret or the engine, or does this actually depend on the tank I am targeting?
If your targetting reticle is very elongated, then don't bother aiming at the top of the tank. It'll increase your risk of missing altogether, and the shot comes in at such an angle that even if you do hit the top of the tank, its chance of penetrating is greatly reduced. So in these cases its best to aim where it maximizes your chance of hitting, as hitting and doing damage is preffered to missing.
Also from over 1000 arty games, the E-50 and IS-7s top turrets/hatches seem to be the most susceptable to top penetrating shots.
What arty tree are you going down?