To be fair there's a definite implied grading of the missing detail in the IFS report.
The Lib Dems probably have the most detailed plans but still have a sum around 1.0% of national income uncosted*. The Lib Dems rely on both larger amounts of tax avoidance income and larger amounts of elimination of fraud from social security spend.
Labour's uncosted gap is the second smallest at 0.4% of national income but while their manifesto does have more detail that Tories, it's open ended on the level of debt and the date of reduction so the IFS have had to make some assumptions. Labour could
The Tories manifesto is basically the opposite of Labour. Unlike Labour they've set out in detail firm commitments as to the speed and scale of the deficit reduction (elimination in the Tories case), but almost no detail at all in terms of how they're going to get there. I'll repeat this from the IFS summary:
But they have not provided anything like complete details of the measures they would implement in order to eliminate the deficit. Beyond current government policies their stated commitments imply:
 An increase in tax revenues of 0.2% of national income, yet their detailed tax
announcements are a net giveaway of 0.1% of national income.
 A reduction in social security spending of 0.6% of national income, yet their detailed
policies bring in only just over a tenth of that amount.
 A reduction in departmental spending of 2.5% of national income, yet their policies
imply (relative to a real freeze) an increase in spending of 0.3% of national income
It might be unreasonable to expect the Conservatives to have set out full details of their
departmental spending plans throughout the next parliament. However, by providing so
much detail on the areas where they plan to increase spending (such as the NHS), while providing no hint of the need for cuts elsewhere, they risk giving a misleading impression
of what public service spending under a Conservative government would look like.
The uncosted* gap for the Tories is 2.3% of national income.
The SNP have the smallest gap 0.3% of national income and provide detail on spend, social security and tax, however they provide no detail at all on what they consider their target acceptable level of deficit. And their actual plans do not match their anti-austerity plans, as although their plans call for small increases over the first 4 years the final year would require a large cut to meet their spending plans. Although the IFS do point out any revenue gained by a crackdown on tax avoidance could be put towards reducing this.
*The IFS considers any revenue relied on by the parties unspecified tax avoidance or form unspecific fraud prevention measures as uncosted. Only the SNP avoided including these sums in their plans. Here's what it says about each parties tax avoidance plans
The Tories
However, by the far the largest tax takeaway is the Conservatives pledge to raise at least £5 billion of revenues by 201718 through anti-avoidance measures. The extent to which this is achievable is highly uncertain, given the lack of information on the tax base in question, and the lack of detail regarding the specific policies that they would
implement. As we have argued before, funding specified tax giveaways that have
relatively certain revenue implications using money from highly uncertain takeaways
elsewhere comes at the cost of adding risk to the public finances.
Labour
On top of this, Labour have also said that they would aim to raise a further £7.5 billion from tax-avoidance measures by the middle of the coming parliament.
Lib Dems
In addition to these specific policies, they also have a target of raising £7 billion by 201718 from clamping down on tax avoidance and evasion, rising to £10 billion by 201920. The extra amount after 201718, which was not explicitly set out in their manifesto, is intended to fund the increase in the personal allowance from £11,600 to £12,500 between 201718 and the end of the parliament. However, like the Conservatives and Labour, they provide relatively little concrete detail of what these anti-avoidance measures would be.
SNP
Unlike the other three parties, the SNP have not factored into their main plans any revenue increase from anti-avoidance measures. Like the other parties, they have said that they believe revenues could be increased further by clamping down on anti avoidance, but have not targeted a specific figure for this nor used any of these
unspecified savings to pay for other policies, which the other parties have. If revenues
could be boosted by clamping down on tax avoidance, this would increase revenues
under the SNP policies above what is shown in Figure 3.1 (and, consequently, reduce
borrowing below what is shown in Figure 2.1 or provide them with scope to cut other
taxes or to increase spending). Overall, this approach to revenues raised from tax
avoidance is more sensible than that employed by the other parties.