"… an amount equal to the rent at which it is estimated the hereditament might reasonably be expected to let from year to year on these three assumptions: (a) the first assumption is that the tenancy begins on the day by reference to which the determination is to be made; (b) the second assumption is that immediately before the tenancy begins the hereditament is in a state of reasonable repair, but excluding from this assumption any repairs which a reasonable landlord would consider uneconomic; (c) the third assumption is that the tenant undertakes to pay all usual tenant's rates and taxes and to bear the cost of the repairs and insurance and the other expenses (if any) necessary to maintain the hereditament in a state to command the rent mentioned above."