Just saw a video indicating that the total of the top and bottom angle on a negative rake scraper should equal 90 degrees. Is this correct?

My scrapers (not negative rake) have an angle of about 65 - 70 degrees, which is about the angle that they were ground to when I bought them. If the above is correct, that would mean I should put a top angle of 30 - 35 degrees on the scrapers to turn them into negative rake scrapers.

Or doesn't the top angle matter much?