"For measuring time at a place which, relatively to the origin of the co-ordinates, has the gravitation potential Φ, we must employ a clock which – when removed to the origin of co-ordinates – goes (1 + Φ/c²) times more slowly than the clock used for measuring time at the origin of co-ordinates." http://relativitybook.com/resources/Einstein_gravity.html This is the sentence just before what you quoted from Einstein. It is not to correct the speed of light. The equation is for correcting clocks in order so that they can both measure the same speed of light. "The principle of the constancy of the velocity of light holds good according to this theory in a different form from that which usually underlies the ordinary theory of relativity."