if you find a tank close enough to a field gun to aim at a certen part or area the spot between the turret and hull is the best spot. when a tank is further out just keep on hitting it.
tigers can be the hardest to bring down so i try not to mess with them, one time i probly spent 2 min bouncing rounds off the side of one and i gave up to fire on a panzer and downed that one with 15 rounds.
panzers take around 25 hits at long range give or take.
M-4's seem to be an easy target, i had one sitting within 100 feet when i jumped into the gun and fired 5-6 rounds and it blew up (same way with the M-8's too)
this should give you an idea on what to expect.