LastAlphaValue=
TOPN( 1
, VALUES( <tableX>[<fieldX>] )
, <tableX>[<fieldX>]
, 0
)

This should work as a measure or a calculated column.

Here's what's happening:

TOPN() returns the top N rows from a table based on a sort order you define. It takes at least 3 arguments:

- N = number of rows to return
- Table from which to return N rows
- Expression to sort by
- (Optional) Sort order: 0 = Descending (default); 1 = Ascending

You can pass multiple sort parameters in odd-numbered arguments after 4, with sort orders on even-numbered arguments after 4.

We are evaluating the top 1 row.

We are taking that row from the table created by VALUES(). VALUES() returns the unique values in a field.

We are sorting by the value of that field.

We are sorting in descending order.