It is a question about math.
func _delta():
var actual = load_id + 1.0
var max_size = to_be_loaded.size()
var percent = (actual / max_size) * 100
actual_percent += 1
$label.text = "Loading... " + str(round(actual_percent)) + "%"
$other_timer.wait_time = interval / percent
$other_timer.start()