graphics unit problem

...thre remaining calculatein [72 points = 1 inch = 25.4 mm] are correct as posted.

Actually the answer depends on the DPI [dots per inch aka pixels per inch] associated with the specific image! Low resolution images [eg for web publication] are typically set to 75-90 DPI Draft printing 150-200 DPI "Quality" priniting 250-350 DPI

72 points = 1 inch = 25.4 mm = 300 document units = 300f pixel units = 300 world units f denotes float