aN independent Scotland would be saddled with debt despite booming oil prices, according to an independent economic consultancy.  While the SNP claims a separate Scotland would have a comfortable budget surplus, limiting the need for public spending cuts, Mackay Consultants suggests it would be about ?2 billion in the red.  Tony Mackay, the firm's managing director, described Scottish government claims as "over-optimistic" and warned that the outlook would become bleak as oil revenues tumble. His comments threaten to undermine the nationalists as they approach May's Holyrood election looking to build support for independence after budget cuts imposed by David Cameron's government.  Last year a report from the SNP administration said a ?10 billion gap between public spending and taxes collected from Scotland would be closed if Scotland received a 91% share of North Sea oil and gas revenues.  This, it said, would have put the country's finances ?1.3 billion in the black in 2008-09.  The nationalists contrast Scotland's position with Britain, which is facing a nearrecord public spending deficit of ?156 billion.  Mackay said it was more realistic to expect that Britain would only allow an independent Scotland an 84% share of North Sea revenues.  Based on figures from the department of energy and climat