Timber may be appraised and sold at a lump-sum value or at a rate per unit of measure which rate may be adjusted during the period of the contract and as therein specified in accordance with formulas or other equivalent specifications for the following reasons:
(a) Variations in lumber or other product value indices between the price index base specified in the contract and the price index actually experienced during the cutting of the timber;
(b) Variance between advertised rates and rates redetermined by appraisal at dates specified in the contract;
(c) Variance between redetermined rates and rates appropriate for changes in costs or selling values subsequent to the rate redetermination which reduce conversion value to less than such redetermined rates; and
(d) Substantial loss of value due to physical deterioration of green timber or other physical damage to the sale area or access to the timber.