Computed column with less size than the "referring column"
Just do a simple cast in your computed column:

create table YourTable
(
    -- your other columns....
    YourCompCol as cast(left(isnull(fieldValue, ''), 500) as nvarchar(500))
)
go

create index IX_CompCol
on YourTable(YourCompCol)
go




Nota Bene

The maximum key length is 900 bytes, and nvarchar(500) has a maximum length of 1000 bytes.  In other words, an INSERT or an UPDATE could potentially fail.