No tolerance value is set in the Regulation, so "common sense" should be used. Rounding at the corner measuring point should be to the nearest centimetre calibration on the measuring tool.
Disclaimer: Please note that the European Commission cannot provide a legally binding interpretation of the EU legislation, as this is the sole competence of the European Court of Justice. Any remarks from the European Commission services are without prejudice to the position the Commission might take should related cases arise in a procedure before the Court of Justice.