It is an interesting issue I'm still not 100% sure that 55% is right, but it does make for an interesting constitutional discussion.

If there is a no confidence vote passed then the government has little chance of continuing in its current formation, but in a hung parliament (including most parliaments in a PR or AV system) it is possible for a minority government to continue until the end of a fixed term parliament.

A coaltion with less than 45% of the seats is highly unlikely to be stable as it would need to support of too many smaller parties on a consistant basis. So if a coalition has less than 45% it can be put out of its misery quickly by a 55% majority, but this should be the last resort of a parliament that cannot provide real government.

Should the existing government coalition fail it is likely that a no confidence vote would follow, but rather than leading directly to fresh elections it would be beneficial to see if there can be a Lib/Lab or Con/Unionist coalition, possibly with the support of one of the smaller parties. But SNP or even Sinn Fein should not be able to force a general election if one of these smaller parties feel like overthrowing a minority government (such as Lib/Lab). This would protect democracy in particular situations but not over-protect democracy.

66% would a workable percentage for the safeguards I am suggesting but could lead to the situation where a minority government (i.e. Conservative on 47% of seats) is able to hold off an election for years because the combined opposition is short by 13% of the super-majority required to call an election. Whilst it might be possible to tolerate this temporarily for a few percentage points short of a majority, this would not be appropriate if the governing party had less than 35% of seats.

In the current parliament it would be possible for the Conservatives to prevent an election because all other parties do not combine to 55%, but having lost a no confidence vote they would need to find a new partner in government or Cameron would be required to step aside and let the Labour leader attempt to form a government (presumably a new alliance would already be formed before a no confidence vote).

So who gains?
Well the Conservatives have an effective veto on elections in this parliament.
Lib Dems can switch their support from Conservatives to a left leaning coalition at any time. They can also stop the Conservatives calling an election when it would be electorally convenient for them.
Labour and other parties almost certainly knows when the next election will be called, and have an opportunity to become part of the government without having to force another election.
Everybody gains because a small party cannot force an election by simply withdrawing support from a much larger party (although each piece of legislation would need the support of more than one party).

I'm sure some of my arguments are self-contradictory, but that is because the whole point is to find a delicate balance. Having thought about this over the last week I come to a personal conclusion (unless I see more compelling evidence) that 55% is about right to balance stability, good governance, and protections to the democratic system which were seriously lacking until this parliament.