Function cube_edge_divisions 

pub fn cube_edge_divisions(radius: f64, chord_tolerance: f64) -> usize

Divisions per cube edge that hold the chord sag of a radius sphere under chord_tolerance. A chord subtending θ sags radius·(1 − cos(θ/2)), so θ = 2·acos(1 − tolerance/radius); the edge arc is 2·atan(1/√2) long.