“Dark Phoenix,” the first X-Men installment and blockbuster-hopeful to be released by Disney since the studio’s completed merger with 20th Century Fox, is touching down in 3,700 theaters. Disney is more cautiously estimating a start between $40 million and $50 million. That would rank as one of the franchise’s lowest openings, in line with 2013’s “The Wolverine” ($53 million), 2000’s “X-Men” ($54 million) and 2011’s “X-Men: First Class” ($55 million). “Dark Phoenix,” the 12th installment in the Marvel-owned series, is a direct sequel to 2016’s “X-Men: Apocalypse.” That film bowed with $65 million and went on to earn $543 million in ticket sales worldwide.