It is difficult to know exactly where the tool is touching the tank, but by doing some light hammering, you can tell by the sound where that point of contact is, then you can move the tank to bring the point of contact to the center of the dent. This a little tricky, but it can be done.

Here's a dynamite trick a seasoned metalworker showed me. When doing work of this sort, he mounts a laser pointer to his ceiling, and aims it at the center of the block he'll be hammering against. Then, when you place the tank over the tool, you can use the point of laser light to position your dent over the tool with precision!