Create a macro that is executed when a cel value chages (not by the user)

Create a macro that is executed when a cel value chages (not by the user)

Ok I have a worksheet "Goal 0" that with some ranges, make some
calculations like...
(in A1) =SUM(G2:G68)*I17
Then if I add/modify any of the values in 62-G68, the cell is auto
calculated (numbers that are mainly negative and some possitive). The
objetive is: According to the sum of the range, find the value of I17
where the result is equal or more than 0. (incrementing it 1 by 1,
decimals not needed)
Manually I can add change i17 untill it reaches the goal. How ever I want
to make it automatically, so if a value in the range of G2 to G68 changes
it recalculate the value of I17, but if already is higger or equal than 0
then do nothing.
Hope I explain it well
Regards