How much space should I allow for future upgrades?

[Pac]

I recently attempted an upgrade from 3.8.1 to 4.0.1 but ran out of space in the middle of the upgrade.

As I didn't know what state this would leave the system in, I restored from a backup I made just before the upgrade, freed up some space on the card and subsequently upgraded successfully.

I know I could move games to USB but I quite like having everything in the same place.
So my question is:
Is it possible to say how much space is a safe amount to leave free on the SD card for future upgrades?

Thanks

Pi Model: (3)
RetroPie Version Used: (3.8.1 upgrading to 4.0.1)
Built From: (SD Image - 8gb card)
USB Devices connected: Logitech K400 Keyboard
Controller used: Wired XBox 360 joy pad
Error messages received: (Not enough space available)
Guide used: (Upgrade WIKI)
File: (N/A)
Emulator: (N/A)
Attachment of config files: (N/A)
How to replicate the problem: Upgrade with too little free space available on SD card

[briankross]

What I've found for the rPi, I like to have double the space necessary for my final file system. For instance, I have about 6GB used on a 16GB micro SD card. Keep in mind if you're recording video and the like then you'll want more space as required.

Generally bigger is better, but the main drawback of a bigger card is the time it takes to create and restore backup images. I started with a 32GB micro SD card and when all testing was done I was only using about 6GB, so I got a 16GB card and revamped the system from scratch. My backups and restorations are faster and more efficient, and the system runs great.

I hope this helps.

[bazmonkey]

Assuming that the upgrade process won't take more space than the size of the base installation itself (e.g. if the base install is 3GB, the compressed packages for upgrading won't be 3GB themselves), and assuming that the increase in the size (post upgrade process) wouldn't be nearly that much... I'd say 4+ GB free is a nice safe amount (and probably overkill).

Given the price of USB flash and microSD cards, I'd whole-heartedly recommend paying some $10 in exchange for quintuple the space and not having to worry about this. I'm particularly partial to Sandisk Ultra Fit drives, solely because they're the size of a wireless keyboard dongle and cheap.