frames for exakt passepartouts

wld be great to have an option to draw exact frames for passepartouts, for example under function 'draw annotations' or under function 'crop' in menue .. with effect to be able to choose exact postion of picture under this frame & ability to calculate borders needed to arrive there before exporting final crop [of course without the drawn passepartout frame] - thx in advance, Nicolas, doing FineArt prints ;)