Jeph said:Multiplying would be more realistic, but it would break the system. Based on the assumption that an additional +2 modifier is twice as strong . . .
Mod . Damage
-4. . x1/4
-3. . x3/8
-2. . x1/2
-1. . x3/4
+0. . x1
+1. . x1.5
+2. . x2
+3. . x3
+4. . x4