Rule – 115A, Rate of exchange for conversion of rupees into foreign currency and reconversion of foreign currency into rupees for the purpose of computation of capital gains under the proviso to clause (a) of sub-section (1) of section 48 of the Income-tax Act,

115A. For the purpose of computing capital gains arising from the transfer of a capital asset being shares in, or debentures of, an Indian company, in the case of an assessee who is a non-resident Indian, the rate of exchange shall be :—

(a) for converting the cost of acquisition of the capital asset, the average of the telegraphic transfer buying rate and telegraphic transfer selling rate of the foreign currency initially utilised in the purchase of the said asset, as on the date of its acquisition;
(b) for converting expenditure incurred wholly and exclusively in connection with the transfer of the capital asset referred to in clause (a), the average of the telegraphic transfer buying rate and telegraphic transfer selling rate of the foreign currency initially utilised in the purchase of the said asset, as on the date of transfer of the capital asset;
(c) for converting the full value of consideration received or accruing as a result of the transfer of the capital asset referred to in clause (a), the average of the telegraphic transfer buying rate and telegraphic transfer selling rate of the foreign currency initially utilised in the purchase of the said asset, as on the date of transfer of the capital asset;
(d) for reconverting capital gains computed in the foreign currency initially utilised in the purchase of the capital asset into rupees, the telegraphic transfer buying rate of such currency, as on the date of transfer of the capital asset.
Explanation : For the purposes of this rule—

(i) “telegraphic transfer buying rate” shall have the same meaning as in the Explanation to rule 26;
(ii) “telegraphic transfer selling rate”, in relation to a foreign currency, means the rate of exchange adopted by the State Bank of India constituted under the State Bank of India Act, 1955 (23 of 1955), for selling such currency where such currency is made available by that bank through telegraphic transfer.]

Reference: www.incometaxindia.gov.in