Here's how to accurately measure the angle of a lathe's topslide. With careful adjustment and a good bit of patience, you can use this method to set the topslide to any required angle.
The topslide is moved a known distance along its axis and a diameter is turned on a piece of scrap material by moving the carriage. Since the topslide is at an angle, the diameters produced will be different. With a little bit of trigonometry, it's easy to work out the angle. Note that the cross-slide must not be moved during this process (well, you can move it, but it must be set back to the same position before each cut).
The shaded part in the drawing below shows the tapered part that the topslide would turn. We can measure both D1 and D2, and we already know L, the distance the topslide is moved. A simple equation then gives the angle. Alternatively, we can work out what difference in diameters a specific angle will produce.
Here's a video of the process. With care, it's easily possible to adjust the angle to within 0.01°. On several different occasions, I had to set the topslide to nominally 1°, and I was able to get angles of 0.996°, 1.010° and 1.005°.