Julie, there is nothing in GSTG specific to your question, our experts advise the following:
If you are using a 40-cell line for the key, you should center the heading according to the 40 cells rather than to the 17 inch width of the paper.
This will put the heading in a more familiar location across the width of the page than if it were shifted farther to the right to be centered on the 17″ width of the page.
We hope this helps!