For the purposes of the covenant mentioned in section 155(3), the landlord may demand such sum as he considers appropriate, up to and including the maximum amount specified in this section.
The maximum amount which may be demanded by the landlord is the discount (if any) to which the tenant was entitled on the making of—
the initial payment,
any interim payment made before the disposal, or
the final payment if so made,
reduced, in each case, by one-fifth for each complete year which has elapsed after the making of the initial payment and before the disposal.