Function frames_to_settle 

pub fn frames_to_settle(segment: &[CaptureImage], settle_frac: f32) -> u32

Index of the first frame in segment whose distance from segment[0] has reached settle_frac of the distance between the first and last frames.

segment[0] is the state at the step and the last entry is the settled state, so the answer is in frames-since-the-step. A segment that never moves (total change at or below the float epsilon) reports 0 — the honest answer for a preset the stimulus does not reach, and the one that keeps StepResponse::ratio finite.