Adding custom column in Power Query

LinlookZDJ 

Try this code, I have broken it into a few columns to try and help make it more clear what I've done: 

let
Source = RawData,
#"Changed Type" = Table.TransformColumnTypes(Source,{{"Start Date Time", type datetime}, {"Complete Date Time", type datetime}, {"Machine", type text}}),
#"Added Index" = Table.AddIndexColumn(#"Changed Type", "Index", 0, 1, Int64.Type),
#"Added PrevCompleteDateTime" = Table.AddColumn(#"Added Index", "Previous Row Complete Date Time", each if [Index] = 0 then null else if #"Added Index"{[Index]-1}[Machine] = [Machine] then #"Added Index"{[Index]-1}[Complete Date Time] else null),
#"Added IsNotLastRow" = Table.AddColumn(#"Added PrevCompleteDateTime", "Is Not Last Row", each if [Index] = List.Max(#"Added Index"[Index]) then null else if #"Added Index"{[Index]+1}[Machine] = [Machine] then 1 else null),
#"Inserted Time Gap" = Table.AddColumn(#"Added IsNotLastRow", "Time Gap", each [Is Not Last Row] * ( [Start Date Time] - [Previous Row Complete Date Time] ), type duration)
in
#"Inserted Time Gap"

LinlookZDJ  

You can use Imke's custom function to improve performance: https://www.thebiccountant.com/2018/07/12/fast-and-easy-way-to-reference-previous-or-next-rows-in-power-query-or-power-bi/ 

I have modified it slightly for you, note this won't give you null for the last row, but will be null for first row of each machine. 

See updated attachment.