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Karl's advice is good. I would also recommend trying LVM2 to give yourself more flexibility in future.

[Ubuntu flavour docs](https://documentation.ubuntu.com/server/explanation/storage/about-lvm/index.html) but it's essentially the same in all distros.

The thing it does for you is not leave you with a partition table that looks like a full board on a sliding block puzzle. You can
1. allocate 70% of a disk,
2. expand that 70% to 80%,
3. take the last 20% for a new partition,
4. then shrink the 80% back to 60% and grow the 20% to 40%.

You have to tell the filesystems before you do it, but this is one of LVM's tricks that a partition table can't do for you. There are also snapshots and other goodies.

Gotchas,
* It's not something you want to migrate an existing install to, unless perhaps when replacing storage hardware.
* You can also give yourself headaches if you use it on removable media. It works but there are extra commands to use.

However if you add a drive, or want to experiment, this is a great tool to know.