Hi John, and welcome to the forum!
You can create custom calculations in a summary by adding another measure then double-clicking the field in the measures list. Activate the Calculation tab, then click the Edit button.
Set your calculation to:
[font="courier"]SUM(GrossProfit)/SUM(Sales) /font[/quote]Change the Display Settings from Default to Custom, and set the format to Percentage. Set one or two decimal places if you'd like.
Now go back to the General tab and uncheck the "Use default title" box, and set the name to something like "Gross Profit %". Click the OK buttons to get back to your summary.