In truth, I find that a tubing cutter with the right (steel) cutting wheel does a much better job than a hacksaw and a guide. (just a little bur inside and out to clean up) When you move on to carbon steerer tubes however, you will have to deep six that and go back to the hacksaw.

You need to be absolutely sure how long the fork must be before you cut. Assemble everything, race, bearings, compresson ring, spacers, stem and the kitchen sink. Get everything in place and tight. Mark, disassemble, double check, think about it.......repeat.....think some more.......and cut.