Inactive shrap damage is 10 by default. The actual inactive shrap damage done by a specific level of shrapnel follows this "formula": Damage = (10 + inactshrapdamage of your ship) * (level - 1). So for a ship with no inactive shrap damage modifiers, the damage done by inactive shrapnel is:
Level 1: 0
Level 2: 10
Level 3: 20
Level 4: 30
If a ship had, say, +3 inactshrapdamage, these values would be:
Level 1: 0
Level 2: 13
Level 3: 26
Level 4: 39
Edit: Note that inactive shrapnel is often the reason a bomb that hits you at high speeds does more damage. By exploding close to your ship's core, most if not all of the inactive shrapnel hits you right away. As for explodepixels, those have been explained well enough by now. The standard (as in, "no items") value is 99. So level 1 = 99, level 2 = 198, level 3 = 297, level 4 = 396.