If we leave the Other category fixed at 9%, and set the Lib-Dems to various fixed positions (I used 2% intervals between 18% and 30%), then we can use the calculator to determine the breakeven points at which Labour or the Conservatives can obtain an outright majority. The breakdown looks like this:
To read this table, assume the Lib-Dems get the percentage given in their column. Then the other two columns give the minimum fraction of total votes needed to secure a majority.
The latest polls have the Lib-Dems with 30-31% of the vote, the Conservatives with 33%, and Labour with 27-28%. That is, both Labour and the Conservatives are around 4% short of a majority. Even if the Liberal-Democrats slide in the polls, either of the major parties would need to capture nearly all of the defecting voters to get a majority. For example, suppose the Lib-Dems slide to 20% by election time. In this scenario, there are 10% up for grabs. To reach a majority at that level, the Conservatives need 40.6%, up 7.6% from their current polling. But Labour would need 35.2%, up 7.2-8.2%. So in either case, the defectors would have to split something like 3:1 for one of the major parties to avoid a hung parliament.
It's worth noting, by the way, that the poll of polls result before the debate shows Conservatives 39%, Labour 32%, Lib-Dems 18%, Other 9%... and the calculator shows that to be a hung parliament as well.