Jackie Zhang didn't stay up late crunching the numbers before he decided that China's Xinyuan Real Estate Co. Ltd. should pay $7.4 million for a portfolio of residential properties in northern Nevada."I'm not a Wall Street guy. Sometimes you judge an opportunity by instinct," says Zhang, the anything-but-reserved managing director of the commercial property department for the Beijing-based company.His instinct, later supported by close financial analysis, led Xinyuan to purchase a portfolio of 325 finished lots and 185 acres of raw land across northern Nevada.The properties, which had been owned by Wells Fargo Bank, extend from Wingfield Springs to Washoe …