Expenditure relating to income not includible in total income is fixed by direct costs plus an investment value allowance, capped by claimed expenditure. If the Assessing Officer is not satisfied with an assessee's claim about expenditure relating to income not includible in total income, the officer shall determine that expenditure as the sum of (i) expenditure directly attributable to such non includible income and (ii) an investment value allowance computed from an annual average of monthly averages of opening and closing balances of investment value, with the aggregate not exceeding the expenditure claimed.