LAGOS — The Dangote Petroleum Refinery has set the minimum subscription for its IPO at 10 shares, or N5,250. The offer of 4.1 billion shares at N525 each runs from 14 September to 13 October and values the refinery at nearly $50 billion. Issuing house executives said they hope to attract 10 million retail investors. A private placement in July raised $2.5 billion.