Do the sums add up?

Yesterday's GERS report on public expenditure and income in Scotland would seem to add some fairly heavy weight to the unionist cause. The Scotsman reports:

So where does the truth lie? Like much else in the debate over independence, it is hard to be definitive.
A nation within a larger state, itself within the European Union, has never before broken away to become a separate country. There are no precedents, and few hard facts.
However, there is at least a base from which to work, in the form of the Government Expenditure and Revenue in Scotland report (GERS), published by the Executive yesterday.
Although the Scottish National Party challenge the assumptions in GERS, most economists and experts on spending accept that it at least provides a basis on which to work.
GERS says that Scotland spends £11 billion more than is brought in by taxes levied north of the Border. Expenditure reduces to £6 billion if all of the tax revenue from oil and gas is assigned to Scotland.

I rather doubt if it is enough for the SNP to rubbish the document by producing higher income figures. They need to show why their income figures are higher and to justify their claims. This kind of argument will simply not do the trick:

The SNP said the report was being used by Labour to "engender fear" over its economic plans for independence. The party produced its own balance sheet for Scotland, "Scotland in Surplus - Past, Present and Future", which claims that for this financial year Scotland is running a surplus of £0.6 billion in absolute terms. The differences between the two figures is largely due to the Nationalists' assuming that a higher proportion of income tax, corporation tax and VAT were raised in Scotland.
Stuart Hosie, the SNP's treasury spokesman, said: "Labour cannot have it both ways. Either their figures are correct, which would mean that after almost ten years of economic mismanagement of Scotland by London Labour, we are in a poor economic position in comparison to all our successful neighbours.
"Or, these statistics are misleading and inaccurate and this is nothing more than an attempt to engender fear in the SNP and our positive arguments for fresh thinking to boost Scotland's economy."
He added: "Our analysis demonstrates that today Scotland has a surplus relative to the UK of £2.8 billion."

But perhaps it is unfair to draw firm conclusions before we have all had time to analyse the GERS report. But this would seem to be a setback for the SNP.