To hold a constant angle...

[Edgepal]

This is a very simple but effective tool that show how many degres you wobble sueingvfreehand sharpening on an benchstone. 

I have use a piece of brass 15x3 mm, a strong magnet and a laser pen fixed on the brass part with a rubber band. The brass part have a angle of 10 degrees, make that angle about 15 degrees.

I also made a "target" for the laser dot. The target shall be 54 cm behind the cutting edge = 1 degree = 1 cm on the target.

Do as you are use to do when you sharpen a knife. Mount my tool on the blade. Put the target behind the sharpener 54 cm from the cutting edge. Adjust "zero" on the target where the red dot are when you sharpen. Now, can you hold a consitent angle the red dot will stay on the zero line...

1 cm up, or down, you wobble 1 degree.

Use a video camera and do not look at the red dot, concentrate on the sharpening process and to hold a consitstent angle. Then, look on the video 

I think you will be amazed 

Thomas