Rackham requires a 1-inch margin throughout your document, but a 2-inch margin for chapter titles and major headings like Dedication and Acknowledgments. Using a Heading 1 style and modifying the style to create the two-inch margin is the best method for doing this.
By adding 1 inch (or 72 points) of extra space above the Heading 1, you combine the default one inch margin from the document with the one inch of extra space on the heading to create your two inches.
Please note that you must have a "Section Break (Next Page)" immediately ahead of your section or chapter heading for the Heading 1 style to insert that additional space. If you're not getting a two inch margin even though your Heading 1 style is correctly set, it's most likely that you're missing the "Section Break (Next Page)". Toggle on the invisible characters to look for that Section Break, and insert it if it's not there. You'll find it in the Layout tab, in the Breaks tool.
See Setting Up a Heading 1 Example on the previous page for the step-by-step instructions for modifying your Heading 1 style.