I was banging my head trying to figure this out and voila ! If you have created a Bucket Field in Salesforce Report and want to reference that Bucket Field in a Formula Field created at the report level (not at the object level),  here is how you can do it.

First of all if you want to refer a field in the Formula field, you need to know the API Name (a.k.a Developer Name). Now the problem is when you create a Bucket Field, all you see is the Bucket Field Name and not the API Name.

(Click on the images below to view the image in its original size)

Salesforce Bucket Field Definition

So to retrieve the API Name for Bucket Field, open the report definition in IDE (eclipse) by double clicking on the report name in left sidebar. Bucket Fields have the API name in the format of BucketField_<9999999> as shown below. In this example the API Name of Bucket Field is BucketField_1023495

Bucket Field API Name

This API Name then can be used in the Formula Field Definition within a report as shown below

Bucket Field in Formula Field

How’s that ?