For the impedance of 129 ohms, using Zo as 50ohms, locate it on the Smith Chart after normalizing it. This is ZLn Later draw a circle, using a compass, that has its pin at the chart's center R=1.0 and its perimeter passes through the ZLn. Finally draw a line that passes through center and ZLn. Extend that line in the opposite direction to intersect with the circle in the lower half of the chart. Find the coordinates of that intersection point. This would be the Y of that line.