How can I force MacOS to make a binary conversion of filesizes in MB, GB...?
Unfortunately, you can't.  See this apple support document which explains that since 10.6 they started measuring storage capacity according to the decimal GB (Gigabyte) rather than binary GiB (Gibibyte) values.  This matches up with how hardware manufacturers measure the capacity of storage devices.  OS X 10.5 and earlier, along with iPhones, iPads and iPods still use binary measurements.