10 to 20
I believe the range 10 to 20 covers the multiplier effect for most objects. You can find the details in any of the government sponsored asteroid impact assessment reports available online. Velocity and mass both matter. Comets usually have a lower density but a higher velocity to give a kinetic energy within a factor of 2 per unit diameter. A nearly tangential impact will also distort a crater from the typical round shape.
Regards,
Mark