FrozenNorth
Hero
A peculiarly specific example.
It is an example that readily comes to mind because: (1) the DCs in the Core rulebook aren’t obviously tied to level; (2) the game seems to rely on the Treat Wounds action to heal up your characters between fights; (3) because a critical success already heals more than a simple success and Expert prof essentially just gives you a +2 to your roll, in many cases it makes for sense to try to go for the critical (and succeed on the lower check) than try for the higher DC and fail.
So with a +4 in the relevant skill, you need Master level (level 7?) before you have a 50% chance of succeeding on your Treat Wounds roll (and can use Assurance).As you can see, the Treat Wounds DC doesn't change (for Expert proficiency), so you'll probably stick a bit longer to 2d8 healing before you aspire to the 2d8+10 level.
Proficiency Rank With (default) Without (variant) Untrained 10 10 Trained 15 15 Expert 20 20 Master 30 25 Legendary 40 30
You are unlikely to ever be able to consistently use Master Treat Wounds. (Druids and Clerics might).