Macquarie Group may have a difficult time maintaining the book value of its holding in Macquarie Infrastructure Group after the directors of the latter yesterday advised that they were likely to write down their own assessment of their value at the end of the year.MIG said yesterday it expected to reduce the value of its tollroad assets to $6.5 billion from a prior book value of $8.6 billion (and from $10.2 billion in the year prior). MIG, like all Macquarie infrastructure offshoots and some other entities, revalues its investments, reports the gains and losses through the profit and loss, and also, where feasible, borrows against those values to pay dividends.Less than a month ago Macquarie Group reported in its profit for the half-year to September 2008 that it held its then 13 per cent interest in MIG at a book value of $854 million, which exceeded the market value by $120 million. Macquarie justified this on the basis of "recent asset sales" by other infrastructure investors.Macquarie Group has until it reports in mid May 2009 for the year to March 2009 to resolve its own approach to valuing MIG.