Well maybe we could define the modifiers:

# of Units Defeated /20 = X # of Attracked Mongoloids * Khan Awe (His modifier for attracting Followers, A Ghengis Khan hero would have an increased # here

That type of forumlaec for the defeat of a province, then for up to 5 (or 4, or whatever you'd like to say) seasons after province taken there would be an additional formula for attracted hordlings.