Negative rake scrapers are ground with a top and bottom angle - and the sum of these is called the included angle. As with any tool grind, different turners prefer different negative rake included angles. However, most agree that an included angle greater than 80º will not perform well, and anything greater than 90º won’t cut at all.
The general idea is that a larger included angle creates a less aggressive tool. On the other hand, a lesser included angle provides a finer finish. Cindy Drozda suggests 75º or less, and Stuart Batty uses a 50º included angle.
Our personal preference is an included angle of 65º (all our negative rake scrapers are ground to this degree). To create this angle, the top angle is ground to 25º, and the bottom to 40º.