Whether you're sharpening your chef's knife on a whetstone or using a honing steel to keep it aligned, the knife's edge should meet the stone or steel at an angle of 22½ degrees.
Imagine that you're trying to slice a very thin piece of the whetstone or knife steel away. The images here (specifically, the red lines) illustrate what a 22½ degree angle looks like. It's about half of a 45 degree angle, which is half of 90 degrees.
You should be able to eyeball it, but these diagrams give you a visual.
It's the same angle for a whetstone or a knife steel. You just have to visualize the angle in a different direction.
Also see: How to Sharpen a Knife