The short answer to the title is about 50%. What they have imposed on Scotland is bad, but not as bad as it could have been; it is EVEL-lite. Under the new system, when English MPs are fairly evenly divided over proposed legislation for England, Scottish MPs’ ability to tip the balance will be halved. They will not be able to impose new laws on England against the wishes of a small majority of English MPs, but they will still be able to vote down any bill which is backed by only a small majority of English MPs.
It seems to me that this version of EVEL could lead to an unintended consequence. Previously, there was a convention that Scottish MPs would not vote on legislation which did not affect Scotland, although this was unenforceable and was occasionally ignored by Labour back in the days when they had a substantial number of Scottish MPs. Under the new rules, there will be a stage in the passage of any specifically English bill (as certified by the Speaker) in which Scottish MPs will be prohibited from voting. (Scottish MPs will, however, be allowed to speak in the Legislative Grand Committee debates.) It can be argued that the new rules supersede the earlier convention, and that by prohibiting voting by Scottish MPs in one specific part of the legislative process for English bills, it implicitly endorses their voting in other stages.
Soon after the last election, proposed legislation, to allow people in England to once again enjoy the sight of a fox being ripped to pieces by a pack of hounds, posed an ethical dilemma for Scottish MPs. Should they vote against such barbarity, or should they let themselves be bound by convention and abstain? If this matter raises its ugly head once more, then I would argue that Scottish MPs can, and should, take the opportunity to speak and vote against any such legislation with clear consciences, thanks to EVEL. If challenged on this, they could always point to the way in which every SNP amendment to the current Scotland bill was voted down by English MPs, very few of whom had even bothered to be present when the amendments were debated.
Of course, if Scottish MPs do succeed in blocking some English legislation, the Tory response might be to go to EVEL-max, and prohibit Scottish MPs from any participation in the debates and voting for English bills, even when these have consequences for Scotland, for example through the Barnett formula. The present version of EVEL has been described as another nail in the coffin of the Union, but it is probably quite a small nail; EVEL-max could be a much bigger nail. Personally, I see nothing wrong with the supporters of independence making the coffin nails and handing them to the unionists for them to hammer in.
EVEL may cause significant problems in the future, as explained in the following quote from an article on the Constitution Unit website (constitution-unit.com/2015/10/23/the-triumph-of-evel-what-next-for-the-english-question/#more-4264).
Another major area of concern … relates to the ill-conceived decision to apply the procedure to Finance Bills, which legislate for taxation such as income tax (on which certain new decisions are expected to be devolved to Scotland)… [B]ecause (unlike most legislation) income tax must be regularly re-approved by parliament in order to remain in force, the provision of a veto to a subset of MPs could potentially allow them to hold the UK government to ransom. Such a scenario is most likely in the event that a UK government lacked a majority in England.
From the same article, another hint of the way EVEL could develop.
Already it has been suggested that Whitehall departments that deal almost exclusively with English policy matters (for example health and education) be labelled more explicitly and accurately as English-focused departments, and this could also be reflected in non-legislative aspects of the Westminster parliament’s work. There are different ways in which the logic of EVEL might in future be applied to other facets of Westminster, including that government ministers appointed to such departments might require the ‘consent’ of MPs representing English constituencies, or that select committees scrutinising such departments be composed only of English MPs.
Perhaps claims that EVEL will make it impossible for for a Scottish MP to become Prime Minister, or be appointed to any major Cabinet position are exaggerated, but it does seem that it will become at least slightly more difficult for Scottish MPs to reach high office, and that this difficulty might well increase in future. If Scottish MPs are not yet second class MPs, the first steps towards that have been taken.
EVEL in its present form is truly a mess – perhaps if the BBC were to make a programme about it they might call it the Great British Mess. The only form of EVEL which might not be a mess would involve the creation of an English Assembly or Parliament – in other words, a federal system for the UK. However, the probability that the UK will develop a proper federal system, with a constitution which is not subject to the whims of whichever government is in power, is vanishingly small. There is only one good way out of the EVEL swamp, and that is Scottish independence – a cure for all EVEL.
PS My apologies to any Welsh people who read this; I have ignored Wales to keep things simple. Sometimes Wales will be included with England, sometimes it will be excluded. Perhaps we should be talking about EVEL/EWVEWL!
PPS If the second word of the title had been evil, my answer to the question would have been higher than 50%.