DIAMOND SHAMROCK <DIA> SETS PRORATION FACTOR Diamond Shamrock Corp said it will accept about 27 pct of the 73,653,000 shares of its common stock tendered in response to the company's offer to pay 17 dlrs a share for 20 mln shares. The company said it expects to mail checks representing the purchase price of the 20 mln shares purchased later this week and will be returning unpurchased shares shortly thereafter.