Less than 100 hours until the polling stations open, and thoughts are naturally turning more and more to just what the result of this election will be. Like everyone else, I’ve been pondering the various post-election deals that are possible and it feels to me that the key number of MPs either Labour or the Conservatives need to win to have a chance at forming a stable Government is around 290.

My thinking’s based on the current numbers for MPs from the other parties being suggested by the various forecasts: an SNP total of 40-50+, Liberal Democrats winning somewhere between 25-30 seats, Northern Irish seats remaining roughly the same, UKIP winning around 3 or 4 and Greens holding their one seat. What I assume is that neither Labour nor the Tories would make a deal with the SNP, but either would make one with the Liberal Democrats. What we can also do is assume that even in the absence of a deal, the other parties are likely to vote in a certain way, especially on issues of confidence.

The reason 290 is key is that it’s the point at which either main party in agreement with the Lib Dems would have enough support to be able to expect to win votes in Parliament regularly. For example, a Labour-Lib Dem government would expect the regular support of the SDLP and (Northern Ireland independent) Sylvia Hermon, and could probably count on Plaid Cymru and the Green’s Caroline Lucas (as well as Alliance’s Naomi Long if she’s re-elected) giving support as well in exchange for some concessions. So, 290 Labour MPs, plus 25-30 Lib Dem MPs and 10-12 others gives a Parliamentary majority without needing any deals with the SNP. Indeed, such an agreement would create a headache for the SNP (and George Galloway): vote against it and they’re voting with the Tories, abstain and they’re doing nothing in Parliament.

On the other side, the same applies but with fewer parties involved. With the support of the DUP, 290 Tories allied with the Lib Dems would have enough seats for a majority in Parliament without the need for any formal deal with UKIP (who’d face the same ‘vote with Labour or do nothing’ dilemma as the SNP in the other scenario) though getting Lib Dems to agree to a deal that formally involved the DUP might be tricky. Indeed, assuming that getting Lib Dems to agree to any deal is a simple matter of getting Clegg on board with it fails to account for the role of the wider party in agreeing it, as Jennie explains here.

They key point here, though, is that for both Labour and the Tories, 290 looks like being the key number of seats to win on Thursday (though that can go up or down depending on how many Lib Dem seats there are – the key figure is having around 315-320 for the two parties combined). If one of them (and the maths suggest it will be only one of them, unless the Scottish polls are way off) can make it there then they will very likely form the core of the next Government, but if neither of them can, then we can expect a long period of coalition negotiation and deal-making before we get a new Government and the shape of it won’t be clear for much longer.