Do I need a guided sharpening system?

[One Armed]

What SFPM range should I shoot for when setting the edge on a dry 400 grit Klingpor j-flex belt (42" if that matters)? Dipping between passes of course, but I was curious if there's a such thing as too slow when setting the initial edge.

No specific speed. Too slow is only if it’s not removing material. Otherwise, go as slow as you wish. Obviously, the slower you go, the longer it will take. But I think you are asking about a safety margin, and the answer is no. You will not hurt anything by going slow. The only tell of TOO SLOW, will be it doesn’t remove material.