Nearest Number Match - Huge Data To Compare

i have two columns in tow seperate workbooks to be compared:

in say inp.xls, there is a column(COL A) containting a lot of entries(numbers) - roughly around 60,000. Now in the other workbook, say mem.xls, i have a smaller set of numbers in a colum(COL B) - roughly around 300.

Now, the problem is, COL B, has numbers which may or may not match with numbers in COL A.

* If there is a match(between COL A and COL B), the row number at the match in COL A.

* If there is no match, the row of the closest number (in COL A) before to the number in COL B.

Example:


ROW COL A-----> COL B---------->
1 2184058252 2184276560
2 2184058280
3 2184058296
4 2184058312
5 2184058320
6 2184773640
7 2184774216
8 2184774272
9 2184774424



Now, i want to find 2184276560 in COLA, since it is not there, i want it to return the row number of the closest(previous) viz. row 5(2184058320).

Now, since this is a huge comparision set, im wondering what is the best way to do it. Im using VBA....