I believe there will always be a little bit of white space. One option, may be to change the print scaling on the Page Setup dialog box to scale to a specified % of size.
Otherwise, it might make a good one for the Ideas section!
I've managed to remove this whitespace for my dashboard:
In File > Print Setup, there's a Layout tab with a section called Page Margin.
Set all values to 0. Then, in the tab Print Scaling, choose "Scale to 100% of normal size".
Finally, go to File>Print to PDF... and pick "unspecified" as paper size.