Dear Lucie ,
There are two ways through which you can create a buckets .One way by using the Bins and other ways through Calculated Field. In your case , I think you should Opt "Calculated Field"
Procedure 1: (Just to make you aware ) - Through Bins
Procedure 2 : Through Calculated Field
Please let me know if you are looking for something else. It would be great if you can post the a sample workbook
Rajeev, this has been really helpful and so simple! I went for the calculated field option in the end. Still a bit more manual but actually quite quick to do! Thanks for your help!