It simply takes experience to learn to estimate distances in the tank gun. Always zoom in all the way so that it looks the same, for comparison sake. If you are using 4 or 5 rounds to sight in an enemy tank, that's not too bad. You will get even better.
As far as ack goes, are you using HE rounds? they allow you to miss by quite a bit. IE, close is good enough on ack.
If ever possible, seek the high ground when shooting at ack and other ground targets. It makes it much easier, even if you are at the extent of your range.
eskimo