Automatic ballista control.

When the game is running, you can view the cursor coordinates.
To do this, you need to switch the game to windowed mode (F4 key).

By default, the program is located on the left side of the screen.
It is best to place the Heroes window slightly to the right of the program.

And regarding the coordinates - everything works fine ;)

P.S. Heroes 3 runs in windowed mode at any resolution of at least 800 x 600.