(ii) The proportion referred to in clause (i) shall be determined by dividing the number of days mainly devoted by such employee or other person for the purposes of the business or profession of the assessee outside India by the total number of days spent by such employee or other person outside India (excluding, in either case, the number of days required for such travel by a reasonably direct route in the mode of travel adopted by him).
Explanation.—For the purpose of this rule, the expression “days mainly devoted by such employee or other person for the purposes of the business or profession of the assessee outside India” shall include any public holiday in a foreign country on which such employee or other person is required to stay outside India, provided that the working days immediately following such public holiday is mainly devoted by him for the purposes of the business or profession of the assessee.
(2) The allowance in respect of expenditure incurred by an assessee in connection with travelling by an employee or any other person within India outside the headquarters of such employee or other person for the purposes of the business or profession of the assessee shall not exceed the aggregate of the amounts computed as hereunder :—
(a) in respect of travel by rail, road, waterway or air, the expenditure actually incurred ;
(b) in respect of any other expenditure (including hotel expenses or allowances paid) in connection with such travel, an amount calculated at the following rates for the period spent outside such headquarters :—(i) where the amount of such expenditure does not exceed Rs. 1,500 per day, the whole of such amount ;
(ii) in any other case, Rs. 1,500 as increased by a sum equal to seventy-five per cent of such expenditure in excess of Rs. 1,500 per day.’
Reference/Source: Check at Income Tax site for latest version of the Rules - link