summary column calcaulation issue

Hello,

If I have a summary column and It calculates the sum of fields

And I create a formula column that reads this summary column with if condition

The issue I have that I want to calculate the fields on the based on the screen

The code is as the following

briefly i want a summary column that calculate the the values of other formula column

how to do this  ?

function CF_sum_VAT_Formula return Number is

begin

          if :qty_1 < 300 then

return(0);

          elsif :qty_1 > 300 then

                   return(:CS_sum_Vat_per_group);

          end if;       

exception

          when others then

           return(null);