Depletion assets shall be valued on the basis of the value left over after deducting from the cost of such assets the depletion charge for each period. Computation of the depletion charge may be made according to one of the following formulas; provided that whichever is used shall not be changed afterwards:
1. To compute at the close of the business year the depletion charge deductible for the current year on the basis of the quantity actually exploited within the current year multiplied by the estimated unit depletion charge, which is worked out by dividing the cost of the depletion assets against the quantity exploitable;
2. To set aside annually from the gross amount of proceeds realized from the exploitation or sale of products the depletion charge according to the Table of Depletion Assets; provided that the depletion charge set aside annually shall not exceed fifty per cent of the amount of gain derived in the current year from the assets before deducting therefrom the depletion charge; and that the aggregation of such depletion charge shall in no case exceed the cost of the assets. In the case of depletion assets producing petroleum or natural gas a depletion in the amount of 27.5 per cent of the gross amount of proceeds realized from sale of the production in the current year may be set aside therefrom annually till the assets are completely exhausted; provided that the depletion charge set aside annually shall not exceed fifty per cent of the amount of gain derived in the current year from the assets before deducting therefrom the depletion charge.