I'm still new to straight razor shaving so I'm no expert. For me (so far) the key is very short strokes around the chin (and jawbone for that matter). Anywhere there are rapid changes needed in the razor to skin angle. On areas like the cheek where my face can be made to be flat I can do longer strokes if I want to. Keeping a consistent angle to my skin with the razor as I round the chin is almost impossible for me. So I watch to see that I like the angle of the razor then do a very short stroke. Then I adjust the angle of the razor and do another very short stroke. The cheeks by comparison can be done with long strokes. My chin always needs a second pass it seems. By shaving my chin/neck area first I can then relather everything and do one pass over the entire face. Of course that means a second pass over the chin and neck under the chin where my beard is the thickest. I hope that made sense.
Jack