I think the attached file has the filter you are after. You were on the right track but needed to amend your Rank table calcualtion slightly.
If you click on the Rank on the columns shelf and choose edit table calculation you will be able to compare this to your original.
All the best,
brilliant thanks - can't believe how close I was! I was amending the rank filter - didn't think to put it on the column shelf.
No problem glad I could help.