I use 300dpi too (exactly 12pixel/mm), and send image using a program which send the print to an online printshop. However you don't have to care about 'dpi' but only of pixels. 75x50cm=9000x6000 pixels. If i order a lot of prints i use jpeg compression, never go under 90% quality. at 89% the file is very smaller than 90% (i don't know why....), but there are some compression artifact. At 90% is ok. For compressing in jpg i use imagemagick (open source) instead of photoshop. I use it also to do high quality downsize to post here and in my website.