Is there a limit to the value that the mount effect can have? I mean, can a unit have mount_effect horse - 100?

How does the mount effect work? I just subtract that value from the unit's attack value to see how it does against horsed units?

If so, can a unit have a negative mount effect with an absolute value that is equal or greater than the units attack value? (e.g. mount_effect horse -12
stat_pri 10 ?)

does the charge value play any role?

Thanks