Budget 2012: Targeting the rich may cost us dear "Those who have got a taste for avoiding the 50p rate may continue to avoid the 45p rate, even if they wouldn't have done so had the 50p rate never existed," Paul Johnson, the IFS director, said. What makes the strategy particularly dangerous, though, is the fact that the Chancellor is already walking a tightrope. According to the OBR, he will meet his fiscal mandate - to eradicate the structural budget deficit within five years - with just £8.7bn to spare. Given the slim headroom, he cannot afford to take in less than the £300m expected from both the new 7pc stamp duty on £2m properties and the £50,000 limit on tax reliefs - both "tycoon" taxes of sorts. At the same time, he needs to be sure that the cost of cutting the top rate to 45p will be just £110m. Worryingly, the IFS warned about all three measures. But its main concerns were reserved for the 45p rate, where just a tiny adjustment in the model used to calculate the revenue pushed up the cost to the exchequer to £1bn. The OBR even suggested that there was particular uncertainty about the costings of three other measures - albeit smaller ones. The last thing Britain needs right now, though, is any more risk. As Fitch Ratings reminded the Chancellor today, the UK's sovereign AAA rating is under threat due to "the very limited fiscal space to absorb further economic shocks in light of such elevated debt levels and a potentially weaker than currently forecast economic recovery." With oil back at $125 a barrel - a 12pc rise since the start of the year and the same punitively high level as last April - and the eurozone's underlying problems still unresolved, the external risks to the economy are bad enough. If the Chancellor wants to keep the bond vigilantes off his back, he can't afford to let tax revenues leak away through avoidance. He needs every penny he can get. However, tax revenues are already coming in below expectations. For the year to April, they are expected to have been £5bn lower than forecast and, for 2013, to come in another £2.8bn below projections. This year, the tax shortfall has been more than made up for by spending cuts, which were £6bn deeper than expected, but few believe that is repeatable. Of the planned 720,000 of public sector job cuts, for example, 270,000 have already been made - a far faster rate of redundancy than expected. IFS deputy director Carl Emmerson even warned that one of the biggest risks facing the public finances is the deliverability of the spending cuts. "The danger is they don't deliver," he said. So far the Government has done very well but so far is not very far. Although a quarter of the £123bn austerity plan has been completed, most of that has come through tax rises - such as the VAT hike. The Government still has to deliver 88pc of the spending cuts - the deepest since the Second World War, the IFS has calculated. Given the risks to the economy from external factors and the difficulties in cutting public spending, the last thing the Chancellor needs is an unreliable tax base. But by switching the focus to the super-rich, who the Treasury estimates will pay an extra £1,300 in tax annually following the Budget changes, that's what he's got.