Where can I find my completion percentage?

Solution 1:

It seems that you can check your game progression on the Ubisoft Club website.

https://club.ubisoft.com/en-GB/game/assassins-creed-odyssey/PC/stats/

On the stats page a column sidebar lists your progression on certain things within the game.

Main story progression

There doesn't seem to be an overall completion percentage though.

Solution 2:

If you're playing through Uplay, you can see a more detailed list under Statistics:

Uplay Asscreed Odyssey Statistics page
(source. the same thread suggests this page can be buggy, however)

Apart from general statistics, it shows the amount of completion in percentages of the Main Progression, Regions Discovered, Reach High Points Synched, Abilities Acquired, and Ship Upgrades.