The world looked very different the last time Scotland had an update on the state of its public finances.

Eighteen months ago, oil was trading at roughly half its current level, and Alex Salmond was a fixture in the BBC Aberdeen studio, railing against the Scottish Executive for a "dodgy dossier" of statistics showing Scotland's "lack of ambition".

Yesterday, with Mr Salmond installed as First Minister, the same statisticians published the latest Government Expenditure and Revenue Scotland figures covering the two years from 2005-07. This time, the SNP happily reported that Scotland - if allocated the tax from oil drilled off its shores - might not be in surplus, but it is in a manageable deficit.

The soaring oil price suggests Scotland will move comfortably out of the red in more recent months, and it is the Nationalists' opponents who are questioning the methodology.

The exercise goes back 16 years, when the Tory government began a calculation of Scotland's contribution to UK taxation and its total public expenditure. That was seen as a means of putting Scotland in its place as a subsidy junkie. The number- crunching has been the subject of fierce political controversy ever since.

But Dr Andrew Goudie, the Scottish Government's chief economist, defiantly claiming his professional autonomy, has been hard at work updating the crunching method.

He acknowledges that there are different ways to do this. Whereas it is relatively easy to account for public money in Ireland or Iceland, the way Britain's public finances work is much more complex and opaque, and it is harder to disentangle any nation's or region's share.

But the St Andrew's House statisticians have given it a major overhaul, re-examining more than 3500 budget lines to see if they can be more accurately attributed to Scotland or the rest of the UK.

The result suggests sloppiness at the Treasury in past years. Even setting aside some relatively minor continuing disputes, the process has found the previous calculation of expenditure has over- estimated Scotland's share by nearly £1bn per year.

The unique status of Scottish Water, decommissioning Dounreay nuclear plant and elements of English justice spending mysteriously counted as Scotland's, are cut down to appropriate size.

On the revenue-raising side, more careful attribution has found another £1bn credited in Scotland's favour, though there remains a health warning over corporation tax.

Jim and Margaret Cuthbert, the retired Edinburgh civil servant couple who have doggedly pursued this issue by prising secret accounting papers out of the Treasury with use of the Freedom of Information Act, can take much credit for this.

Pleased with their handiwork and increased clarity, they pointed yesterday to some telling findings about the state of Scotland.

Its share of income tax take reflects relatively low income. If Scotland earned at the same level as the UK average, it would be paying more than £1bn extra in income tax. The share of stamp duty paid is much higher per English person than per Scot.

But Scots' payment of tobacco duty - nearly £1bn in 2006-07 - soars above its population share. Alcohol and gambling tax run substantially ahead of England's per capita share as well.

That, however, is unlikely to be the main use of these statistics. They are already a potent new weapon to be deployed in the political warfare over Scotland's constitutional future.

Nationalists can be confident the next two sets of figures, due in June each year, will put Scotland comfortably into the Brent crude black.

Takings from this year's oil bonanza will be perfectly timed to fuel their case, with publication only four months before the target date for an independence referendum.