Good day, Ladies and Gentlemen!
Just to reassure myself: As far as I read in here, Ubuntu uses the correct decimal base interpretation of the MB prefix. So, in Nautilus, a file of 1MB is actually really 103 KB (1000 kilobytes) and 1GB = 103 MB (1000 megabytes).
However, I want to know if the same is true during the installation of Ubuntu, when creating the partitions manually. Am I right to enter 20000MB if I want a 20GB partition?