Where a deduction from rent payable to a landlord is made under this Schedule, and the landlord himself holds the premises under a lease for a term of which less than 20 years is unexpired, the landlord is entitled to deduct from any rent payable by him under the lease such proportion of the amount deducted from the rent payable to him as the rent so payable by him bears to the rent payable to him, and so on in succession with respect to every landlord holding the premises for a term of which less than 20 years remains unexpired and who is both receiving and liable to pay rent in respect thereof.