Dont believe everything you read on Sheldon.
So you want to install 50mm of spacer and Gunnar recommends a max of 40mm. OK, you can either cheat an install an extra 20mm of spacers. Get a stem with more meat on the bottom or get a stem with more rise to it.
How can a stem be recommended by your fitter if its doest give you the proper rise with the recommended spacers? Sounds to me that your fitter deosnt have the proper stem in stock.