SQL Server Check for IsNull and for Zero

set @SomeVariable = @AnotherVariable /
(case when isnull(@VariableEqualToZero, 0) = 0 then 1 else
@VariableEqualToZero end) - 1

If you're using SQL Server, you can probably use a NULLIF statement?
i.e. set the value to NULL if it's 0 then set it to 1 if it's NULL - should catch for both 0's and NULLs:

SET @SomeVariable = @AnotherVariable/ISNULL(NULLIF(@VariableEqualToZero,0),1) - 1

SET @SomeVariable = @AnotherVariable / COALESCE(
        CASE 
             WHEN @VariableEqualToZero = 0 THEN 1
             ELSE @VariableEqualToZero
        END, 1) - 1