I suggest you create at least 3 partitions:
/ and /home and a swap partition
If it were my system I would probably allocate the following:
This assumes everything except your /home partition will be saved on / (or a child of /)
/ = ~36GB
/home = ~120GB
swap = ~2X available RAM (if you have 2GB ram create a ~4GB swap partition)
It is simple and allows you to persist your /home data even if you install a different distribution.
This is always up for debate. It depends on how the system will be used.
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