If you "flip" your range values so that they're in ascending order rather
than descending order:

=IF(F20<G21,0,MATCH(F20,G21:G30)/100)

Format as Percentage

If you place your percentages in column "H" next to their corresponding
values in "G", then you can simply use a VLOOKUP.  If you use "TRUE" for the
range_lookup, Excel will lookup the closest value that is greater than or
equal to the value that you are searching for.  Just make sure that  column G
is sorted from smallest to largest.  For example, you could use  
VLOOKUP(F20,$G:$H,2,TRUE) for the following data:

G      H

0      0%
5      1%
10    2%
20    3%
30    4%
40    5%
50    6%
60    7%
75    8%
100  9%

There is no limit here, so you can add as many parameters as you like up to
the row limit of your version of Excel.  You can create the reverse effect
(<=) by sorting from largest to smallest in column "G".

-BN