The IFS's David Phillips has a post at the Future of the UK and Scotland site, which like the post by my colleagues David Bell and David Eiser at Scottish Fiscal & Economic Sudies, describes the Byzantine, labyrinthine, systems needed to implement a system that maintains the Barnett Formula whilst implementing significant devolution of revenue powers.
There is a simpler way to do things: declare some expenditure items as UK responsibilities, with the others as devolved; declare a set of UK taxes to pay for these UK expenditures, and allow the devolved administrations to raise taxes to meet their responsibilities. The UK expenditure responsibilities would include UK debt repayments, any equalisation payments that were agreed, and fiscal transfers in response to business cycle fluctuations and asymmetric shocks.
The first step to implementing such a system (which might have a chance of being stable in the long term) is for the UK Government to recognise that it wears (at least) two hats: as the sole level of government responsible for UK functions; and as the government of England (and also the government of Wales & Northern Ireland for those areas in which Scotland has devolution but they don't). It needs to know what hat it is wearing at any one time, and what policy levers are associated with that hat.
In David Phillips' post he notes: "But what if the UK government wanted to spend more money on defence or state pensions, or wanted to increase taxes to reduce borrowing. ... One interpretation of the Smith proposals would be that the UK government could not use an increase in income tax – one of the main taxes it levies – to fund these policies. This would be absurd and the Treasury has informed us that it was not the intention of the Smith Commission to constrain the UK government in this way."
Contrary to David Phillips, I don't think that a recognition that some taxes are for funding devolved services, whilst others fund UK services, is absurd. On the contrary, it seems like a pre-requisite for a functioning system. Note also that such a recognition would not preclude the use of income tax for the funding of UK services: for example in my submission to the Smith Commission [*], I recommended that the UK Government have as one of its revenue streams the top rate of income tax (because such a tax base is likely highly mobile in response to differences in the rate). But there would have to be two income taxes: one levied by the UK government for UK expenditure, and one levied by devolved government (which in England may well be just the UK government wearing its English government hat) for devolved expenditure.
But in any case, the first step towards a workable system is for some self-awareness on the part of HMT.
[*] Shorter, and better written version is Chapter 8 of 'Beyond Smith' ebook