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#11
I always initially install onto a small partition, one which I create using the installation media, first having deleted all pre-existing partitions on the drive. So the clean install always begins with unallocated space. The reason for 20Gb is perhaps a carry over from when Windows would/could throw install files way out onto a drive, in some cases enough to limit shrinking the drive beyond a certain point.
As soon as the initial part of the install is done the partition is expanded to 70Gb (my chosen size).
A clean install for me, and with all the programs you see listed in the above procedure installed comes in at around 27Gb. This figure also includes all my personal files.