If the amplitude of a symbol is Si then the power is proportional to the distance from the origin to the constellation point
power = |Si|^2 / 2
Therefore if you want a signal with quarter power
power / 4 = |Si_new|^2 / 2
|Si_new| = (power / 2)^0.5 = (|Si|^2 / 4)^0.5 = |Si| / 2
Thus the magnitude of the new signal should be half the magnitude of the old signal
