How about this equation:
$shadow_Pointx = $watermark_Pointx + ($shadowDistance/100 * $watermark_x * sin($shadowAngle))
$shadow_Pointy = $watermark_Pointy + ($shadowDistance/100 * $watermark_y * cos($shadowAngle))
By assuming 90 degree will set the shadow on the right side of the watermark, I guess the result should be 960,140..