PointsToScreenPixelsX and PointsToScreenPixelsY are Excel methods of a Window, or a Pane.
The values returned are pixel values; to set a Form's position, you must convert the pixel values to Points. See: [
^].
You can find
what appears to be a complete solution for your question here: [
^].
Caution: I do not have Excel on this machine, and cannot test this using C# automation.