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I also run an E3D with 0.4mm nozzle. Typically you want your extrusion width to be at least 10% larger than your nozzle diameter (so minimum extrusion width of 0.44mm). I run my extrusion width at 0.5mm because it makes designing components easier for perfect wall thicknesses. Your 0.4mm nozzle will have no problem with a 0.5mm extrusion width.