How to properly align Kivy/KivyMD widgets

I developed a GUI with KivyMD. It consists of eight box layout arranged vertically in another Box Layout. I started adding checkbox, labels and DropDownItem. Everything appears aligned except for the DropDownItem in the second row. I tried with pos_hint but I cannot move it along the x axis. I would like that the DropDownItem begins where the first checkbox of row1 begins.

Here is the full Kivy kv code:

<InitializeComponent>:
   orientation: 'vertical'
   padding: 20
   
   MDBoxLayout:
      MDLabel:
         text: 'row 1'
      MDCheckbox:
      MDLabel:
         text: 'Item 1'
      MDCheckbox:
      MDLabel:
         text: 'Item 2'
      MDCheckbox:
      MDLabel:
         text: 'Item 3'
      MDCheckbox:
      MDLabel:
         text: 'Item 4'
      Widget:
         size_hint_x: None
         width: myCombo.width
      Widget:
         size_hint_x: None
         width: myCombo.width
      
           
   MDBoxLayout:
      MDLabel:
         text: 'row 2___________' 
      MDDropDownItem:
         text: 'select'
         pos_hint: {"left":1, "center_y":0.5}
      Widget:    
      Widget:
      Widget:
      Widget:
      Widget:
      Widget:
      Widget:
      MDDropDownItem:
         id: myCombo
         text: 'select'
         pos_hint: {"center_y":0.5}
      MDDropDownItem:
         id: myCombo
         text: 'select'
         pos_hint: {"center_y":0.5}
          
   
   MDBoxLayout:
      MDLabel:
         text: 'row____3'   
      MDCheckbox:
      MDLabel:
         text: 'Item 5'
      MDCheckbox:
      MDLabel:
         text: 'Item 6'
      MDCheckbox:
      MDLabel:
         text: 'Item 7'
      Widget:
      Widget:
      MDDropDownItem:
         text: 'select'
         pos_hint: {"center_y":0.5}
      MDDropDownItem:
         text: 'select'
         pos_hint: {"center_y":0.5}   
   
   
   MDBoxLayout:
      MDLabel:
         text: 'row____4_____'   
      MDCheckbox:
      MDLabel:
         text: 'Item 8'
      Widget:
      Widget:
      Widget:
      Widget:
      Widget:
      Widget:
      MDDropDownItem:
         text: 'select'
         pos_hint: {"center_y":0.5}
      MDDropDownItem:
         text: 'select'
         pos_hint: {"center_y":0.5}
         
   
   MDBoxLayout:
      MDLabel:
         text: 'row____5'   
      MDCheckbox:
      MDLabel:
         text: 'Item 9'
      MDCheckbox:
      MDLabel:
         text: 'Item 10'
      Widget:
      Widget:
      Widget:
      Widget:
      MDDropDownItem:
         text: 'select'
         pos_hint: {"center_y":0.5}
      MDDropDownItem:
         text: 'select'
         pos_hint: {"center_y":0.5}
           
        
   MDBoxLayout:
      MDLabel:
         text: 'row____6'   
      MDCheckbox:
      MDLabel:
         text: 'Item 11'
      Widget:
      Widget:
      Widget:
      Widget:
      Widget:
      Widget:
      MDDropDownItem:
         text: 'select'
         pos_hint: {"center_y":0.5}
      MDDropDownItem:
         text: 'select'
         pos_hint: {"center_y":0.5}
            
         
   MDBoxLayout:
      MDLabel:
         text: 'row____7_______'   
      MDCheckbox:
      MDLabel:
         text: 'Item 12'
      Widget:
      Widget:
      Widget:
      Widget:
      Widget:
      Widget:
      MDDropDownItem:
         text: 'select'
         pos_hint: {"center_y":0.5}
      MDDropDownItem:
         text: 'select'
         pos_hint: {"center_y":0.5}
          
   
   MDBoxLayout:
      MDRaisedButton:  
         text: 'BUTTON'    
         size_hint: 1, None
         pos_hint: {"center_x":0.5, "center_y&开发者_开发问答quot;:0.5}

Any tip to keep the widgets aligned?