The formula would be
col mean( If( :depth == 20,:CD,.), :Wafer, :Lot ID)
Since only 1 row will be found to have :Depth=20, Col Mean() could also be substituted with Col Sum() etc.
One could also subset all of the Depth=20 rows into a new data table, and then Change in the subsetted data table the column CD columns name to CD(20 mm) and then using
Tables=>Update
join the subsetted data back into the original data table, matching on Wafer and Lot ID
Jim