This simple calculation causes more correspondence to Electoral Calculus than any other. Here's how we do it.
The majority is defined as the difference between the number of seats that the Government has and the number of seats that all the other parties have. It is not the difference between the number of seats that the Government has and half the number of seats in parliament. For example, if one party gets 340 seats out of 650, then the other parties have 310 and the majority is thirty seats, not fifteen. Not everyone believes that so Electoral Calculus set up a vote on Twitter, which had a clear majority for the correct answer.
Then the question of the Speaker arises. There is one Speaker, Lindsay Hoyle, who was originally elected as a Labour MP. There will also be three deputy speakers, two Conservative and one Labour, who conventionally do not vote either. So, counting Sir Lindsay as Labour for this purpose, the four non-voting MPs cancel each other out and do not affect the majority. As long as you include the speaker as belonging to his/her former party, the majority calculation is unaffected by the Speaker and deputy speakers.
Finally, there is the Sinn Fein party from Northern Ireland. It will have around six to ten MPs and they have historically not attended the House of Commons and not taken part in parliamentary votes. If they continue to do that, then the effective government majority will increase because the number of opposition MPs has decreased. But there is no guarantee that Sinn Fein will never attend in future. If it was in their interests to do so because, say, it could result in a united Ireland, then there is nothing to prevent them from choosing to attend. As such we do not assume that Sinn Fein will necessarily always be absent, and the majority is not adjusted for that.
With the current prediction of 348 seats for the Conservatives (including two deputy speakers), then the opposition MPs (including the Speaker, one deputy speaker and Sinn Fein) would have 302 seats, giving a Conservative majority of 46 seats. On those days where, say, the seven, Sinn Fein MPs do not attend, then the majority would be 53.