Doing a FINDITEM() on blank values is inefficient as the function has to traverse the entire list before it returns a blank.
If the majority of the values are not blank, then check for BLANK first:
IF ISNOTBLANK(Line Item) THEN FINDITEM(List, line item) ELSE BLANK
If the majority are NOT blank:
If ISNOTBLANK(Line Item) THEN FINDITEM(List, line item) ELSE BLANK
If the majority are blank:
If ISBLANK(Line Item) THEN BLANK else FINDITEM(List, line item)
Depending on the the number of blanks the line item could have will dictate which formula to use. If there are more blanks than not, then use the second formula. If Line Item is more dense (has more values than not) then the first formula should be used. If there will never be blanks in the values, there is no need to check for blanks: