ok, I have solved it by storing the center point of selection rectangle as a reference point and calculated then in "cursorMoved()" the offset and set then the new values by
Qt Code: 
  1. setValue(old_marker_pos.x() + offset_x, old_marker_pos.y() + offset_y)
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It has somehow still strange behaviour. I think I have to calculate the intermediate point between the selected items to have better behaviour or do you have another idea?