Function slack_estimate 

pub fn slack_estimate(
    sum: f32,
    squares: f32,
    spread: Spread,
    effect: f32,
) -> f32

The same free time as slack, by the Fenton–Wilkinson approximation of the same model (Fenton 1960; TE18): a lognormal matched to the mean and variance of the day’s total, the shared day effect included. Cheap, from two sums of the steps’ medians (sum = Σ m, squares = Σ m²): the plan checks with it, step by step, that a day holds its steps and their free time; the free time it keeps is then slack’s, on what it laid.