Henry,
The angle you create with the file can be (and I would argue should be) slightly lower than the angle you use to finish up with the stones. The file is used to hog off the steel so that the stones can do the job they're supposed to do, which is polish. This is called a duplex bevel and will always result in a stronger cutting edge no matter what angle range we're talking about. Even my knives are done like this because I want them to stay sharp as long as possible.
Sharpening a single bevel is actually easier than a double once you understand the principle but, I'm too biased to offer my recommendation of a particular brand of sharpener.