You just need to minus the Round amount from GrandTotal Amount.

But Formatting supports not to show + sign it may show you either 0.30 also -0.30 as positive and negative value from Custom format. I used The below Question to Make RoundAmount calculation as per your requirement Just check it and implement your Formula in your code as per your logic.

Rounding Off formula field values In Crystal Reports using C# in ASP.Net

I have created RoundAmount as Formula to calculate Round Amount and added in same Report also added actual amount from Field to show the difference.

Formula

Round (CCur({DataTable1.Freight}),0) - CCur({DataTable1.Freight})

Formatting Formula Field to Report.

Output