Hello, can you help me please

Cell B1 contains a complex mathematical formula which requires (in several
places) a number from cell A1.

Cell A1 can contain any integer number, but it is usually the same (30).

I would like to be able to leave cell A1 empty, and only enter a number when
it is not 30 ,

ie the default value of A1 is 30, unless specified.

How do I go about this ?

Should I look at conditional formatting, or put lots of IF functions into an
already complex formula?

Thank as always

KK