Am not sure if its applicable, but on the stage on the metallurgical microscope I use, I keep a block of polystyrene weighted down with a steel bar. The objective to the stage is dead 90*, so I just stab the blade into the block at whatever angle I need to get as much of the edge in focus at the same time. This is also a very good method for determining the real apex angle after stropping procedures or just freehand sharpening. A white background underneath is a good idea, or can use a small flashlight with a cloth etc draped over it. Backlighting the edge is a great way to increase usable depth info, especially at higher magnification.