According to the
document submitted to the Planning Department (page 44), each residential floor is 9 feet, 8 inches in height (this diagram seems to be of the shorter tower):
It looks like this diagram has 36 main floors, and then 2 mechanical floors. If you only consider the 36 floors, and say they're all exactly 9.67 feet, you get 348 feet. Level 2 is shown as 11.67 feet, so that brings it up to 350 feet, ignoring the extra height on the ground floor.
So, the 350' number must exclude the mechanical penthouse, and probably counts the 0' level as midblock or some place that gives them a few extra feet to work with.