Function scale_on 

pub fn scale_on(
    scale: Scale,
    (width, height): (u16, u16),
    space: &Size,
) -> Factor

The scale scale gives frame (width, height) on space (the display inside its safe area): for Scale::Integer, the largest whole number that fits, which is 0 when even 1 doesn’t; for Scale::Fit, the largest factor that fits.