(1)Subsection (3) applies—
(a)for the purpose of making a comparison under rule 4 of section 838(3) if section 838 has effect because of section 835(1), and
(b)for the purpose of making a comparison under section 839(3) if section 839 has effect because of section 835(2).
(2)In this section “the actual lease” means the lease mentioned in section 835(1)(b) or (2)(b).
(3)The commercial rent is the rent which might be expected to be paid under a lease, of the land in respect of which the payment mentioned in section 835(1)(b) or (2)(b) is made, which—
(a)was negotiated in the open market when the actual lease was created,
(b)is of the same duration as the actual lease,
(c)is subject to the terms and conditions of the actual lease as respects liability for maintenance and repairs, and
(d)provides for rent payable at uniform intervals and at an appropriate rate.
(4)Rent is payable at an appropriate rate if—
(a)it is payable at a uniform rate, or
(b)in a case where the rent payable under the actual lease is rent at a progressive rate (and such that the amount of rent payable for a year is never less than the amount payable for a previous year), it progresses by gradations proportionate to those provided by the actual lease.