JOHN Swinney yesterday repeated his warning that the Scottish Government will block the new Scotland Bill unless a deal is done on the so-called 'fiscal framework' that underpins it.

Giving evidence to the House of Commons Scottish Affairs Committee in Perth, he said negotiations with the UK Government were reaching "the end of the road".

If agreement is not reached by February 12, he warned, Bill will not be approved before the Holyrood election.

For Mr Swinney's opponents, this amounted to a piece of all-too-predictable political brinkmanship.

Ian Murray, Labour's shadow Scottish Secretary accused him of making "continued attempts to manufacture grievance".

It's impossible to tell whether Mr Swinney is rattling his sabre because he wants to extract a better deal or, as some opponents suspect, because he doesn't really want a deal at all. Control over income tax, they reason, would make it far harder for a Holyrood administration to blame Westminster for the state of its finances.

Most of Mr Swinney's opponents are not quite that cynical and believe a deal can still be done with a little give and take.

Enough muttering has escaped the behind-closed-doors talks for us to have a pretty clear idea where the stumbling block lies: how to cut the Scottish Government's annual block grant from the Treasury (which has to happen because income tax raised in Scotland will no longer go to the Treasury).

Jim Gallagher, the academic and former Better Together adviser, goes into considerable - algebraic, even - detail about it all in a new paper published today by Nuffield College, Oxford.

Mr Swinney's favoured method for readjusting the block grant is known as "per capita indexation," which would protect the Scottish Government's budget in future years if Scottish tax revenues rose more slowly than in the rest of the UK because of slower population growth.

However, Professor Gallagher argues the system would be unfair to the rest of the UK because it would transfer too much income tax from England.

"It would be hard to explain to English taxpayers that if their population grows they must provide services for them, but also send some of the income tax these voters pay to support additional services in Scotland when population has not grown there," he writes.

Professor Gallagher also rejects a second proposed mechanism for cutting Holyrood's grant, known as the "levels" method.

Under the system, the Scottish Government would probably have to impose higher taxes to maintain the same spending levels as in the rest of the UK because of Scotland's smaller tax base, and that's probably not in the spirit of the Smith Commission, whose recommendations set the Scotland Bill ball rolling, he argues.

He believes a third system being discussed by the two governments, known as "indexed deduction," would allow fairer levels of English income tax to be transferred to Scotland.

Writing in The Herald last year Professor Anton Muscatelli, the head of Glasgow University, identified the "per capita" mechanism as potentially providing hundreds of millions of pounds more for the Scottish Government, over a period of years, compared with the other methods of recalculating the budget.

Is it that or nothing for Mr Swinney? We'll soon find out.