What swing do the Conservatives/Labour need to win an overall majority?
On a purely uniform swing, the Conservatives need to be 11.1 percentage points ahead of Labour to get an overall majority, that is, they need a swing of 2% from Labour compared to where they were at the last general election. For Labour to win an overall majority they need to be 2.6 percentage points ahead of the Conservatives; that is, they need a swing of 4.9% from where they were at the last general election. If the Conservatives are 4 points ahead of Labour then the two parties would have an equal number of seats.
Of course, in reality the swing is not uniform. There is significant random variation from seat to seat, there are also some systemic difference - Scotland, for example, often displays very different shifts in the vote to England & Wales, while past elections suggest that MPs first elected in 2010 will perform above the national swing at their first election as incumbents (especially those who defeated an incumbent MP from a rival party to win their seat).
More importantly, an estimate based on a pure uniform swing assumes that the level of support for the Liberal Democrats remains unchanged from the last election, something that few (if anyone) expect to happen. If there is a drop in Liberal Democrat support, then the lead that the Conservatives would need to win an overall majority comes down. For example, if the Liberal Democrats managed 15% at the next general election - much higher than the level of support most current opinion polls are showing - the Conservatives could win with a lead of about 7 points over Labour and no significant Lab to Con swing.
Which seat do the Conservatives/Labour need to win an overall majority?
This was actually the one question I was asked more than any other in the run up to the last general election, mostly by journalists who wanted to do a vox pox or a feature in the seat that the Tories "needed to win" in order to have a majority. The correct answer is, of course, that there is no one seat that either party needs in order to win the election, swings are far from perfectly uniform, so if the Conservatives fail to win the seat that is, on paper, the 20th most winnable there is no reason they couldn't win the seat that is the 21st most winnable.
The Conservatives won 306 seats at the last general election, so to win an overall majority they need to gain an extra 20 seats. The seat with the 20th smallest majority over the Conservatives is Middlesbrough South and East Cleveland....but this isn't necessarily the 20th most winnable - the Conservatives may feel they have better chances for gains elsewhere, and better prospects in Liberal Democrat held seats than in Labour ones.
The Labour party meanwhile won 258 seats at the last general election, so to win an overall majority they need to gain an extra 68 seats. The seat with the 68th smallest majority over Labour is High Peak... but once again, this isn't necessarily the 68th most winnable seat for Labour.
Are you adding Prospective Parliamentary candidates for each seat?
Before the 2010 election I added party candidates to each constituency as they were selected... or at least, attempted to. Tracking all 4000 or so candidates and providing photos and biogs for them was, as I should probably have realised, an almost impossible task for one person to do - especially for independent and minor party candidates who would often be announced mid-way through the Parliament, but fail to actually contest the election. I hope to add confirmed candidates in 2015 and maintain lists of candidates selected by the main parties - but I won't be attempting the impossible again.
Seat X is actually far more winnable than seat Y
Yes, it probably is. The lists are target seats are based purely on percentage majorities, in reality the demographics of particular seats, or past tactical considerations mean that the raw figures give a misleading impression of how winnable a seat is. Every seat has a comments section where you can add your own opinions about how winnable they are.