He wants a 6x8" Photo I'd say :) Not so abnormal size for a 4:3 Crop sensor.
Simply put, make a 300dpi canvas the same resolution as the original photo.
Using CM in PS to get what you think it should be on the screen is wrong wrong wrong. As different screens have different size pixels. Always make it the largest it will go and scale DOWN. Don't start small.
I may be looking at this to simply, but if you just want a 6cm x 8cm print of an image why not just designate that particular print size in the print options? Just go to file>print with preview...and you can set the height and width by cm in the print dialogue (provided the image is the correct proportions you should be able to get that exact size). It's irrelevant how big it is on the screen. Of course a calibrated printer helps if you are doing something for commercial purposes.
Always be yourself, unless you can be Batman, then always be the Batman