No need to adjust the range, just put the sights right in the middle of the tank. It should blow up in just a couple of shots. Most of the time tanks are very vulnerable between the road wheels or just above the tracks. Take it easy and look for blasts and smoke puffs on impact. The tutorial mission also tells you when you hit the tank.
You'll know if you produced damage to the tank depending on the type of impact is displayed.
If you just get a plink noise with a puff of gray smoke, it usually means that round bounced off and did nothing.
If you see a large flash/explosion and a different type of noise, you penetrated the armour and should continue to focus on that area you shot. Remember that location in the future for when you are online and faced with a tank. Keep in mind that different angles produce different results, which means if you're on a different angle and shooting the same spot as before, it might not penetrate and will have to find another location.
It takes practice to know where all the sweet spots are.
If you think a specific spot has weaker armour than another spot, give it a shot and see. Sometimes you'll have to shoot at all kinds of different spots on the tank before you find a good area you can get a round through..... you can even be lucky with a shot and kill someone inside the tank. If you can take out the commander in the tank, sometimes that will make the tank automatically explode.