Case C-570/15. Free Movement – Social Insurance

What standard or standards should be used to assess what legislation is designated by Regulation (EEC) No 1408/71 as applicable in the case of a worker residing in Belgium who performs the bulk of his work for his Dutch employer in the Netherlands, and in addition performs 6.5 per cent of that work in Belgium in the year in question, at home and with clients, without there being a fixed pattern and without any agreement having been made with his employer with regard to the performance of work in Belgium?