MTOW_approach_speed x SQRT (actual_weight / MTOW).
So in this case, 63 x SQRT(0.8) = 56.3, which I rounded up to 57.
It's just an extension of L=W=½RhoV²SCl. So, as Cl is the same (and a function of AoA), the speed has to go with the square root of weight, to balance the equation.
G