pocketmine\entity\Human::getLevelXpRequirement PHP Method

    public static function getLevelXpRequirement(int $level) : int
    {
        if ($level <= 16) {
            return 2 * $level + 7;
        } elseif ($level <= 31) {
            return 5 * $level - 38;
        } elseif ($level <= 21863) {
            return 9 * $level - 158;
        }
        return PHP_INT_MAX;
    }

 public function setExp($exp)
 {
     $this->progress = $exp / Human::getLevelXpRequirement($this->level);
 }