Assuming you have a list named ListA
18.523
19.015
20.648
and you wish to look up either
19.014 or 19.016, named Near,
and get the same answer, 19.015,
then you can use this array formula
=SUMPRODUCT(--(MIN(ABS(ListA-Near))=ABS(ListA-Near))*ListA)
entered with Ctrl+Shift+Enter.