public static function getFactor($source, $target)
    {
        $source = self::standardizeUnit($source);
        $target = self::standardizeUnit($target);
        $factors = static::getFlipCascadeFactors();
        if (isset($factors[$source])) {
            [$to, $factor] = $factors[$source];
            if ($to === $target) {
                return $factor;
            }
        }

        return null;
    }