You would then have to put in place something to prevent lords from kicking knights out of an estate that they want to leave empty.
Why? If efficiency goes above the 75% max when a knight takes an estate, the lord actually gets
more when a knight takes the estate.
The 75% efficiency max is
before lord share is counted. Even in empty estates the lord should only get at most 50% of the total taxes (max lord share). Thus, if right now, making a 100% efficient empty estate produces 100 gold, and the lord gets 50 gold, after the 75% max is added, the lord would only get 38 gold, and when a knight takes the estate, the lord, provided lord share is 50%, would then get 50 gold once again. Beneficial to both parties.