macOS installation fails due to apparent lack of space...

Disk Utility or Terminal are the only accurate ways to determine the free space available on your drive(s). The report from Storage is not to be relied upon for accuracy.

Your startup drive apparently has only 40GB of free space, as reported by Disk Utility.

Ideally you'll always want to keep a minimum of 10%-15% of the total startup drive capacity free for the OS to run properly.

You will need to delete files or relocate them to another external drive.

Here is some of Apple's guidance on the subject: Free up storage space on Mac - Apple Support

Yup, Used space registers in Disk Utilities as 444.45 Used Space

Follow the advise from my colleague @D. I Johnson 👍

Now, before you asked about Purgeable Space

Purgeable Space is controlled by the Operating System and not the user 

Get detailed information about a disk in Disk Utility on Mac

From another contributor @etresoft regarding Free Space and Available Space 

Free vs available disk space huge differe… - Apple Community

Quote >>  “ The "available" storage is the amount of used storage that the operating system could automatically delete if it felt that it was really necessary. The "free" storage is the amount that you can actually use for something.

There are system processes that run in the background and automatically delete some of the "available" storage and convert it to "free". If you completely run out of storage, then those system processes will try a little harder. When you "delete" files you are just hinting to the operating system that you don't need those files anymore. The operating system will eventually remove them, but on its own schedule.

Certain tools will allow you to force the issue and manually clean up some of this storage and manually delete local snapshots. But that is only temporary. "  << End Quote