Why not just use image size dialogue? Load image into PS, go to Image -> Image Size (Alt-Crl-I), untick "Resample", put 300 into resolution field and there you are.
And yes, almost NO client is capable to understand the relationship dpi<->pixel<->print size. This has been in sum waisting about 3 days in my life explaning to different clients.
Another classic is "But, but... Our print service said It has to be 300dpi !" - for a 8x4m construction sign for example...
Good luck!