Using Excel 2003

Using the following formula
It works great Except that when the input cell is blank ( or "deleted"), a
Zero
is entered in cell "W?"
This in turn causes the formula to return a 30, NOT the Zero that I'm
Looking for.
Can the formula be reworked? or somehow multiple nested?
(the input/supply cell is restricted to minimum of 24 and maximum of 72,
so that part is looked after)

=IF(W58=24,24,IF(W58<=30,30,IF(W58<=36,36,IF(W58<= 42,42,IF(W58<=48,48,IF(W58<=54,54,IF(W58<=60,60,IF (W58<=66,66,72))))))))

Thanks
--
John F. Scholten