The score rating variance is based on percentage of time above or below the par time, up to 40%.
Keys timed: Base rating += (Margin / 40) * 5
Keys untimed: Minimum(Base rating, 170) -= 5 + ((Margin / 40) * 5)
You completed Iron Docks 18 with 9.9% under time, so your score was accordingly boosted by an additional (9.9/40)= 24.75% of a single key level (5).
I’ve finished a +25 Junkyard with 40.8% additonal time, and have earned the maximum score possible off of that specific key level - 175 plus 5 for the bonus time.
For keys that failed to time, you can still earn points if you finish within 40% of the par time.
EDIT:
Untimed keys now have an effective ceiling at +20, or 170 base rating.