New Custom Column Conditionally Set based on Existing Column and Rows

I'm trying create a new column.  The new column should equal the existing column "Value" if the existing column "Adder" field is blank, but if it's not blank it should equal Value of current row - Value of row matching CaseID, Year, Month, Iteration, and where the value in Input_Name = the value in Adder.  For Example, in the new column for row 1, the new column value should be -0.088 (rounded)....taking the value - the corresponding value of the input name Henry Hub since that's the description in Adder. In row 7 it should simply = the value in the Value field 2.5494...