Running total by group within advanced editor

Hi,

I've managed to do running totals by grouped data in power query by performing the process on one of the grouped tables then converting to a function
and invoking on within the original.

So,
< (GroupTotal) =>
let
Index = Table.AddIndexColumn( GroupTotal , "Index" , 1, 1, Int64.Type),
buffer = Index[Units] ,
Output = Table.AddColumn ( Index ,"RTC" , each List.Sum (List.FirstN( buffer , [ Index ] ) ) )
in
Output >

Rather than having this a function in itself, I've seen a couple of tutorials where it is added in the advanced editor to the grouped data,
but I keep getting it wrong.
Any reading recommendations welcome, I find a lot of YouTube vids a bit hard as if not familiar with syntax it's easy to miss somthing.
I've attached the sheets I've been playing around with.

Richard

Hi,
thanks I'll try that.

RD

Hi Glenn,
the out put is the grouped rows, as in the attached file, it's just that I don't want to achieve this through invoking a custom function but rather embed the function with in the code through advanced editor.

Richard.

Hi Oly,
I've been doing some online reading and I wonder if you can help, when creating a query by having performed the steps and then manipulate the code in the editor if usually start with stating a parameter ( x ) then the go too => and on the next line 'let' and then the body text starts.
I other case the when tying one in from scratch it starts within the 'let' and 'in' and then there are cases where as above there is a let followed by another let and two 'in' , and I find it a bit confusing.
Can you direct me to a good source of information.
Richard.