24. That used to be the kind of number of Conservative seats Labour needed to win at the election to ensure it ended up the largest party in parliament.

But that was before the rise of the SNP. Now Labour needs to win anything up to 45 Conservative seats in order to be the largest party and get first shot at trying to form a new government.

Here’s how the maths might work.

For the sake of argument I’ve assumed the Tories and Labour will win roughly the same number of seats from the Liberal Democrats, so neither party will have a net advantage. I’m also assuming Ukip will end up with a minimum of two seats won by the Tories in 2010.

This gives the Tories an initial 46-seat lead over Labour. It follows that for a one-seat lead over the Tories, Labour needs to gain a minimum of 24 seats.

But to maintain this one-seat lead, every loss Labour makes to the SNP necessitates further Tory gains. This is how the arithmetic then unfolds:

Of course, if the Tories lose more seats to Ukip, or fail to pick up as many Lib Dem seats as those gained by Labour, then Labour’s task becomes a little easier. Conversely if a few Lib Dem seats elude Labour’s grasp, or Ukip even manages to take a couple of Labour seats, the numbers become even tougher for Ed Miliband.

As it is, he faces a stiff challenge in simply trying to stay level-pegging with the Conservatives. Even losing half of Labour’s current tally of Scottish seats might require around 35 gains from the Conservatives. If the more extreme opinion polls prove accurate and Labour is virtually wiped out in Scotland, Miliband will need well over 40 gains from the Tories. That’s the kind of haul which seemed feasible a year or so ago, but perhaps not now.

From a realistic 24 to a dizzying 45: two numbers that sum up why this election has become such a challenge for Labour.

(NB: I updated this post a few hours after it was first published to clarify some of those all-important  statistics.)