Knockback should be based on the static friction of the object and be relative to weight and the amount of of surface in contact with the ground. So a standing human is relatively easy to knock around, while the same human lying on the ground is very difficult to move.
Basic knockback value is HP/2 for living things, HP/4 for unliving things, and HP/8 for homogeneous things. Halve that value for things that only have one small point of contact with the ground (like a man standing on one leg), double it for things with 4 points of contact (like a quadraped or a crawling man), and triple it for things with much of their body touching the ground (like a blob or prone man). An object moves 1 yard for every full multiple of its adjusted knockback value.
- Mr. Remarkable kicks a HMMWV jeep for 10d of damage, totaling 37 crushing damage. A HMMWV has 72 homogeneous HP, giving it a basic knockback value of 9. With 4 wheels, it's basically a quadruped, and its effective knockback value is 18. Mr. Remarkable's kick shifts it 2 yards.
- Hauiroto the living tornado slams into an HP 20 zombie, doing 42 points of knockback. The zombie has a basic knockback value of HP/4, or 5, and is standing up, which neither increases or decreases the basic value. The zombie flies 8 yards.