Power Query - get column name

I don't quite follow. You need to specify "Job Name" once so the function knows what column you want to read text lengths from. There's no other place in what I suggested that has "Job Name" hard coded and the output is what you specified.

Maybe if I rewrite it a bit more like your code it will be slightly clearer?

(R as record, MyColumn as text) as text =>               //R = row 1, MyColumn = "Job Name"
let
    Text = Record.Field(R, MyColumn),                    //Returns R[Job Name] = "Controller"
    Source = Text.Length(Text),                          //Returns length("Controller") = 10
    NewSource = Number.ToText(Source),                   //Returns "10"
    FinalText = Text.Combine({MyColumn, NewSource}, " ") // "Job Name" & " " & "10"
in
    FinalText                                            // "Job Name 10"