In this article written by Hitman, you can read that not only the U-552 implement this tactic. There are also references in logbooks of U-505.
Determining the speed from the revolutions account provided only a general idea about the speed of a ship.
http://www.u-historia.com/uhistoria/.../intecnico.htm
The article is called "Adquisición de datos para las soluciones de tiro en los U-boot", in the technical section. The page does not let me give the direct link.
Best Regards.