For filing of quarterly ITR for individuals earning purely from practice of profession who opted for 8% income tax, how can the P250,000.00 be deducted. Is it gross receipts for the quarter less P62,500 (250,000/4) equals taxable income?

There is no need to divide the amount of P250,000.00 allowed as reduction into four quarters. The said amount was considered in the design of the revised quarterly income tax return (Form 1701 Q) which reflect a cummulative quarterly computation.