Possible to make B14 to become B13, B12 and so forth when dragging the formula down?e

I don't know what values you have or much else about your worksheet. This however might work for you. It does count backwards from B14 as you fill the formula down the column.
Formula with commas

Formula:

=(1/(2*B20))*SUMPRODUCT(OFFSET($B$2:INDIRECT("B"&(ROWS($E$2:E2)-1)*-1+14),0,0)-OFFSET($B$2:INDIRECT("B"&(ROWS($E$2:E2)-1)*-1+14),F3,0),OFFSET($B$2:INDIRECT("B"&(ROWS($E$2:E2)-1)*-1+14),0,0)-OFFSET($B$2:INDIRECT("B"&(ROWS($E$2:E2)-1)*-1+14),F3,0))

Formula with semi colons

Formula:

=(1/(2*B20))*SUMPRODUCT(OFFSET($B$2:INDIRECT("B"&(ROWS($E$2:E2)-1)*-1+14);0;0)-OFFSET($B$2:INDIRECT("B"&(ROWS($E$2:E2)-1)*-1+14);F3;0);OFFSET($B$2:INDIRECT("B"&(ROWS($E$2:E2)-1)*-1+14);0;0)-OFFSET($B$2:INDIRECT("B"&(ROWS($E$2:E2)-1)*-1+14);F3;0))