You might prefer higher or lower values than that. In practice, I find that value much too high, and I get better results with values closer to 0.008mm^3. In theory, the value should be equal to lineWidth^2 * layerHeight (not using circle math because Cura calculates extrusion as if it is rectangular), so for 0.42mm line width and 0.2mm layers, it should be 0.035mm^3. You can work around this defect in Cura by setting Retraction Extra Prime Amount. This would cause the assumption of "adding plastic to existing plastic" to be true for the entire length of the line. What a slicer should do is dwell at the start location and extrude enough plastic to lay down a full line width before moving further. This underextrusion is extremely slight, however, and you already compensated for it when tuning your extrusion anyway.)* (Teeeechnically, the *entire line ends up underextruded because the printer never deposits the full required amount of filament to achieve the configured line width, thus it is always playing catch-up. Thus, you get underextrusion at that location. This assumption is correct for most of the distance covered while printing each continuous line of filament, but it is not true at the beginning of the line, because it's starting a new line where there is no plastic at all. The rate of extrusion assumes that the nozzle is adding plastic to existing plastic. When starting a new line (not just after a Z travel, but any travel), Cura starts both extrusion and motion at the same time. I believe Cura (and possibly all slicers) makes an assumption that is fundamentally wrong. If you get your filament temperature, extrusion ratio, retraction distance/speed, and PA/LA tuned really well, you will probably find that Cura's seams get significantly worse, not better.
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