shear force at boundary

We all know that $$\tau = VQ/ It$$ how to determine the shear stress at G ? I'm having problem of finding Ay centroid if the solid that i found earlier is y = 98.5mm

so , my working is Q = Ay = (250x10^-3)(50x10^-3)( 50+125-98)(10^-3) = 9.63x10^-4

I'm considering the area above the G .

Is my concept correct ? edited : There's a dashed line from G to the bottom . It's making me confused . for Q , should i consider the area either to the left or right of the G ?