Adding math
 

I am trying to do a strange ways of adding, like if cell is a1=500, b1=500,
c1=500 then on cell f1 has column down to f5 with formula like first cell
f1=65, f2=100 on down to f5, then next roll over g1 to g5 has formula of
subtract of f1 from a1 which is 435, then g2 has f2 subtract from a1 then its
335, now ? is if g4 show 0 or -0 I want it to pick up b1 from a1 and keep on
subtracting till last cell is completed like g5 is last cell with out going
pass 0 or -20