Add Custom column based on string in another column (if device = "iPhone" or "iPad" then do this)?

PattemManohar 

A-HA! :) so that is it... this works sort of like recording a Macro and then just the "raw" M language is copied and shared. 

In that case, how do I use this? This is what I did:

1. I opened my document, clicked the Query name in the right side panel;
2. I chose Query > Edit;
3. In the Power query Editor I selected the "Add custom column";
4. Pasted Greg_Deckler code into it and added the column;

This did produce the small table with FR and IT results. However, in mz initial post I simpliffied the problem mz creating this simple dataset. The real data that I have is some 20 fields and 15.000 records. Greg's code works for the example but not for my real life case.  

Generically, this is what I need: 

• create custom field (called "Corrected Total sales");
• by adding two already existing fields ("Website conversion value" and "Mobile app conversions value");
• based on the values in a third field (if "Impression device" equals {iPhone, iPad, iPod}... then divide /2, otherwise add normally);

This is what was already in there in the steps I managed to do myself:

let
    Source = Csv.Document(File.Contents("C:\Users\Ed Chigliak\Google Drive\XXXXX\XXXXXXX\reports\raw-data\fb\XXXXXXXXXX-FR-IT-ES-XXXXXXX-Ad-sets-1-May-201814-October-2018.csv"),[Delimiter=",", Columns=28, Encoding=1252, QuoteStyle=QuoteStyle.None]),
    #"Promoted Headers" = Table.PromoteHeaders(Source, [PromoteAllScalars=true]),
    #"Changed Type" = Table.TransformColumnTypes(#"Promoted Headers",{{"Reporting starts", type date}, {"Reporting ends", type date}, {"Ad set name", type text}, {"Platform", type text}, {"Placement", type text}, {"Device platform", type text}, {"Impression Device", type text}, {"Campaign name", type text}, {"Delivery", type text}, {"Date created", type date}, {"Budget", Int64.Type}, {"Budget Type", type text}, {"Amount spent (GBP)", type number}, {"Website conversion value", type text}, {"Mobile app purchases conversion value", type number}, {"Website conversions", Int64.Type}, {"Mobile app purchases", Int64.Type}, {"Website purchase ROAS (return on advertising spend)", type text}, {"Mobile app purchase ROAS (return on advertising spend)", type number}, {"Results", Int64.Type}, {"Result indicator", type text}, {"Cost per results", type number}, {"Reach", Int64.Type}, {"Impressions", Int64.Type}, {"Link clicks", Int64.Type}, {"Landing page views", Int64.Type}, {"Ends", type text}, {"Starts", type date}})
in
    #"Changed Type"

Cheers!