>How do you write an index expression where the first field needs to be descending, but the second may need to be ascending?

The other entries are possible solutions -- but it seems to be a bit cumbersome to store in the index. Also this makes using rushmore more difficult with this index. Instead of transforming the field in the index, define a new field of the same size and do the transformation of the value there. The field assignment can be done by a stored procedure on record update/add trigger. Then just build the index with this transformed field. Rushmore should still work -- just define a variable to get the transformed value and then use this in the query expression.