You need at least 3 identifiable features. One at the start of the track deviation, one for the off track fix and the third at the track regain fix although you could DR the last one with some loss of certainty of position.
Agree, but as an aside, have you ever tried flying with a blank sheet, with just the geometery drawn on it? As a way of proving that what is underneath doesn't matter it's amazing!
The simple maths, when applied carefully, is brilliant!