In the CoH world, it was two frontal shots.
On a side note: I almost believe relic is too scared for casual players to implement drastically different front and rear armor values (or even side armor for that matter), because flanking would become a necessity. (For example the Tiger has 300 front armor and 250 rear armor)
thats not really a fair example. the tiger has the best rear armor in the game and also the best rear to front armor ratio. its rear armor is 5/6 of the front while most vehicles have 1/2. quite a few tanks even have much less than 1/2, such as the brummbar, panther, kv2 and elefant. i guess its a matter of opinion, but i think 1/2 is already a pretty drastic difference. lowering it further wouldnt even have an effect in most cases since the majority of tanks/AT will already pen rear armor most of the time.