Currently (r59) it is necessary to limit the processed area manually to suitable values.
The attached patch sets the MinX/MinY/MinZ/MaxX/MaxY/MaxZ values to reasonable values after loading a file. By default a margin of 10% compared to width/depth/heigth of the loaded model is defined.
"MinY" will not go below zero, if the complete model is at a non-negative heigth.
Values, that are read from a config file, overwrite these guesses.