When converting DPI/PPI to mm for printing purposes you should have a minimum of 12 pixels per mm. See the attached scale for printing the perfect image. This is just for reference against your own calculations. The left column follows the format and the mm size for your print and then you follow across the pixel dimensions to supply the printer. Most users/printers would prefer the size without bleed and they normally add 5 or 10 mm of paper/stock so that the print can be easily mounted. So For example I normally print 2:3 the ratio of my sensor with print size 37.5 x 25 ( 10 x 15 inches ) This means that according to the scale that at 300 dpi resolution I need pixel dimensions of 2953 x 4429. You can also use an online calculator like the one here PIXEL CALCULATOR