If you extend the line from COM to O outwards beyond O you find it is normal (at 90 degrees) to the flat surface of the hemisphere. Since the surface is at 60 degrees to the vertical this means that the angle the line COM to O makes with the vertical must be

90 -60 degrees = 30 degrees.

The horizontal distance is then multiplied by the distance you calculated (which is the hypotenuse of the triangle).

Hence the answer is as you said 1.125 cm.

Does that help?