如何在Kivy的滚动视图中定位屏幕中央的标签？

from kivy.app import App
from kivy.uix.scrollview import ScrollView
from kivy.uix.gridlayout import GridLayout
from kivy.uix.label import Label
from kivy.uix.floatlayout import FloatLayout

class CustomScrollView(ScrollView):
    def __init__(self, **kwargs):
        super(CustomScrollView, self).__init__(**kwargs)
        self.layout = GridLayout(cols=1, spacing=10, size_hint_y=None)
        self.layout.bind(minimum_height=self.layout.setter('height'))
        self.add_widget(self.layout)

class MyApp(App):
    def build(self):
        scroll_view = CustomScrollView()
        label = Label(text='屏幕中央的标签', size_hint=(None, None), size=(400, 400))
        layout = FloatLayout(size_hint=(None, None), size=(800, 800))
        layout.add_widget(label)
        scroll_view.layout.add_widget(layout)
        return scroll_view

if __name__ == '__main__':
    MyApp().run()