Here's a breakdown of how using -5/+10 affects average damage, when you deal an average damage of X, and need to roll a minimum of Y on the die to hit (N.B. I'll be going lower than 2 because while the chance for success without -5/+10 ceases to change, the chance for success with it continues to change). The value you get is the increase, so a negative means -5/+10 does less damage.
Y
| -3 | -2 | -1 | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 |
Dmg Diff
| 9.5 | 9-0.05X | 8.5-0.1X | 8-0.15X | 7.5-0.2X | 7-0.25X | 6.5-0.25X | 6-0.25X | 5.5-0.25X | 5-0.25X | 4.5-0.25X | 4-0.25X | 3.5-0.25X | 3-0.25X | 2.5-0.25X | 2-0.25X | 1.5-0.25X | 1-0.25X | 0.5-0.25X | 0.5-0.2X | 0.5-0.15X | 0.5-0.1X | 0.5-0.05X | 0.5 |