Needed...formula for fixed total....

THIS IS AN INVENTORY SITUATION. I AM LOOKING TO HAVE A FIXED RUNNING TOTAL IN COLUMN "E" (QUANTITY ON HAND). i WANT TO ABLE TO CLEAR THE ADDED INFORMATION IN COLUMN "F" THROUGH "S" (SALES AND PO'S IN), BUT THE LAST TOTAL CALCULATED IN COLUMN "E" REMAIN, EVEN IF I CLEAR THE CELLS AND ADD NEW WHOLE NUMBERS. I WANT TO KEEP ADDING OR SUBTRACTING TO THE NUMBER ALREADY THERE IN COLUMN "E". THE FORMULA i HAVE NOW IS SIMPLE "SUM". IN COLUMN E TO SUBTRACT FROM THE COLUMN F-O ACROSS AND ADD FROM COLUMN S. WHICH WOULD WORK GREAT IF THE TOTAL IN COLUMN E STAYED WITH THE LAST NUMBER CALCULATED... I WANTED IT TO WORK LIKE THE YTD TOTALS...IF THIS MAKES ANY SENSE ...TO MAINTAIN THE VALUE OF QUANTITY ON HAND. ATTACHED IS MY FILE THAT I CREATED..GREATLY APPRECIATE ANY HELP....MAKE WHATEVER CHANGES NEED BE AND UPLOAD AGAIN...