Michel, thanks for posting a good question and a packaged workbook!
So what you seem to really want to do is use Measure Values/Names in a calculated field. Unfortunately this cannot currently be done. If you want to vote for the Idea (feature request) for this here's the link: http://community.tableau.com/ideas/1187
The workaround is of course reshaping your data, but you don't want to do this. You might be able to do something using a Custom SQL connect, but that's beyond me expertise. Maybe one of the other folks can help with this.
In my case, i want the user to select 2 Time ranges (Year, Quarter or Month) and compare Success over Opportunities for the selected periods.
The success is already a calculated field ( registered + took a trial, etc...) and opportunities is also a calculated field ( Visits - AlreadyRegistered - AlreadyOnTrial, etc...).
So when i wrote that i didn`t want to reshape the data, i had in mind to calculate the values in Sql before bringing them in Tableau, which is not possible because i want the user to be able to select the Time ranges.
But after checking the SQL thread (http://community.tableau.com/thread/127506), i went with Jonathan Drummey`s ideas of adding a column in my query, in order to create a dimension with the names of the Measures i want to display. Since i have three Measures, i have to triple the Sql query and Extract, but for this report i can afford it.
But for sure the Idea is definitely a must and I added my vote on it.