That's right -- it's the same note an octave down. And yes, that's a trick you can use to figure out intervals going down instead of up (people to learn intervals going up first, so that's easier).
The "formula" would be something like this, to figure out the complementary interval in the opposite direction:
1) Subtract the interval from 9 (so 4th -> 5th, 2nd -> 7th, etc).
2) Flip the interval type if needed:
diminished -> augmented
minor -> major
major -> minor
perfect -> perfect
augmented -> diminished
Examples:
major 3rd up from C = E
minor 6th down from C = E
perfect 4th down from G = D
perfect 5th up from G = D
dimished 7th up from C# = Bb
augmented 2nd down from C# = Bb
Of course, once you've analysed/built enough downward intervals, you'll start recognizing them without needing any tricks, and that'll be even faster... make sure you don't jump into the "shortcuts" every time without ever trying it out normally, or if might take you longer to reach that stage.
-Rob