Balckbird, I would love to take credit for the marker trick, but I can't. My mentor in the ski shop turned me on to it.
dshack, those extruded aluminum file holders are for people with a lot of experience and time on their hands. You need to be very careful that you ALWAYS apply the same amount of light pressure to the file or you will change the effective angle of the file with respect to the edge you are filing.
The key to setting side edge angles is to use ONE tool for both setting the desired angle and maintaining it. The only requirementa are that the tool very securly hold the files and diamond stones that you will be using and that it is stable as it glides across your base. I have settled on the "Side of Beast" tool (available here) because it is easy to change the angles, it clears filings so they fall on the floor instead on onto your base, it has a largege surface area that rides on base making it stable, and it holds files and diamond stones rock solid.
There are other good tools out there as well, much is personal preference for the feel of the tool (not yours ) in your hand. Be leary of tools that use ball bearings or pins (like some swix tools) as they focus your pressure on the base and tend to put dents in it - especially as you are learning.