Which software are you using and in which mode do you see the error? I mean constant velocity or exact stop mode or both consistently? New cutter with 0.4mm error in 3mm is a lot, but I have received cutters with that much error from the factory, brand new. Of course, cheap eBay stuff but I complained and the seller sent me a batch of new ones and those were good. I don't know if 2-flute cutters are more accurate than single flute, but I guess they are easier to make and to measure, so I prefer using 2-flute or more, but I don't cut ply or any other wood.