Useful blade angle trick
Here is a way to get the correct angle when sharpening a knife.
Measure the width of the blade at the handle end. Do this in millimeters, if possible (makes the math easier).
For 20 degrees, divide by 3
For 15 degrees, divide by 4.
That is the height you should hold the back edge, at the handle end, above the stone to get the desired angle.
A new, unshuffled , deck of Bicycle playing cards has 3.5. cards per millimeter. Do the math and count out the number of cards to get the height.