The thing to remember with boring tools is that,. being relatively small, they deflect.
hence as the bore approaches finished size, you start to take spring cuts.
You will be surprised just how much a 6 or 8 mm bar deflects under cutting forces..
Often it will take more than one spring cut before the tool stops cutting (Hence the need to withdraw the tool with the work stationary, to avoid a helical back cut.
Again, stiffness in tooling and work increases accuracy.
Howard