“Finally (and most importantly), when the estimated aggregate rack-rental value of the demised premises ...” [and there he was taking the expression from the lease itself to which he added the observation] “(whatever that may mean) has been agreed or assessed by the valuer in accordance with the provisions in that behalf contained in the lease, the landlord does not obtain the benefit of the whole (or anything like it) of the difference between that and the original rent but only half.”