When you partition with ext4, it has a journal which helps with file recovery. But journal takes space. Also Ubuntu typically reserves 5% that you do not really see unless you start calculating sizes, so system will not run out of space and crash.
What I have not seen anyone post is how to add nomodeset for nVidia or other video issues with UEFI. With BIOS it is hit any key, then f6 and choose.
Someone posted this, but each vendor's UEFI may be different, I think it is after an install to add grub to the UEFI menu, if it did not install correctly:
Enter your UEFI menu, select "Boot maintenance manager", then "Boot options", then "Add boot option", then "NO VOLUME LABEL,....Primary,Slave...1, GPT,..", then browse the /EFI/ubuntu/ folder via the UEFI boot menu, and select the grubx64.efi . Give it the name you want (eg "Precise"), then "Commit Changes and exit", then Enter.