More wealthy would-be buyers are doing the math, and deciding their deals for a piece of New York City real estate aren’t adding up. More of the city’s wealthiest residents are opting to rent, according to data from UrbanDigs released earlier this summer. Between January and April, 300 ultra-luxury rentals — defined as homes asking more than $20,000 a month — were rented, a 30 percent uptick compared to the same period last year. UrbanDigs’ findings are proof of a trend that’s...