People seem to be getting good results with just the manufacture recommended mix of 209. Then again it is a bit of a ghetto solution for people needing a flat edge when they don't already have one and are unwilling to spend hundreds/thousands to get a straight edge.

The only other way I know of getting a cheap flat edge is the method used for hand grinding optical flats, (people hand grinding their own telescope mirrors need tolerances that make machinists look like cowboys) the downside of that method is it means weeks and weeks of boring repetitive manual labour. By the time you factor in the cost of all different grades of grit and building a jig it would work out more expensive then letting gravity do the work.