Assuming you enter a tolerance level (+/-) in a separate cell for what an acceptable difference in intensity (say 0.001 in K1), then try:

=IF(C2=".",".",IF(MAX(ABS(IF(IF(ISNUMBER($C$4023:$C$4071),$C$4023:$C$4071-C2<=$K$1),$C$4023:$C$4071)))=0,".",IFERROR(ABS(B2-INDEX($B$4023:$B$4071,MATCH(MAX(ABS(IF(IF(ISNUMBER($C$4023:$C$4071),$C$4023:$C$4071-C2<=$K$1),$C$4023:$C$4071))),ABS(IF(IF(ISNUMBER($C$4023:$C$4071),$C$4023:$C$4071-C2<=$K$1),$C$4023:$C$4071)),0))),".")))

confirmed with CTRL+SHIFT+ENTER not just ENTER, then copied down to just before the US list.

## Bookmarks