I did a little check on exactly how much affinity affects the damage.
Turns out positive affinity gives a 40 % damage boost compared to neutral while negative affinity does 20 % less damage compared to neutral affinity.
If we compare to negative affinity, an attack with neutral affinity does 25 % more damage and an attack with positive affinity does 75 % more damage.
Compared to positive affinity, an attack with neutral affinity does 28,6 % less damage (divide by 1,4) and an attack with negative affinity does about 43 % less damage (divide by 1,75).
This is pretty basic but knowing the exact numbers make other calculations easier for those of us who like to try to figure out some of the game mechanics.