Power Query Variables and how to use them

Hi,
I recently had to use a variable in power query , I could not get it to accept the column name ,but haven't been able to find much information on how to use them. I had to declare the start date (sdate) as a variable to use with List.Select, but then had to also make the rest a variable also up to that point it had just been ? whatever not being a VAR is ?
Work book attached ; queries 'typed in ' and "VAR steps" .

     

	


















	
 Table.AddColumn( ddate , "new date " ,each   
let Mdate = [Sdate] , 
change = 
List.Distinct( 
List.Select(  
 List.InsertRange( 
List.Transform( 
 { Number.From([Sdate]) .. Number.From([End Date]) },
(A)=> Date.StartOfMonth( Date.From(A) ) ),0,{ [Sdate]}  ),
      each Number.From(_) >= Number.From(Mdate) )) 
 in  change)


 

I tried using ;

let Mdate = [Sdate] , 
   Table.AddColumn( ddate , "new date " ,each
List.Distinct( 
List.Select(  
 List.InsertRange( 
List.Transform( 
 { Number.From([Sdate]) .. Number.From([End Date]) },
(A)=> Date.StartOfMonth( Date.From(A) ) ),0,{ [Sdate]}  ),
      each Number.From(_) >= Number.From(Mdate) )) 
   )

but this didn't work, can someone help with information on why I had to use the variable in the first place and any general help

Richard