For starters, maths-wise, if you lose one MP to the opposition your majority goes down by -2.
Now I don't know where exactly the -51 comes from. Politicshome puts it at -43 as of 9 hours ago.
The roll-call of 21 rebel Conservatives from whom the whip has been removed includes [...] Their expulsion leaves Boris Johnson 43 votes short of a majority.
That makes sense since the majority was -1 (319 to 320) right before those expulsions, but after the defection of Phillip Lee to the Lib Dems.
Yesterday, the Parliament website actually listed 320 MPs as Conservatives+DUP (310+10). So subtraction-wise, they did the same math since yesterday, subtracting 21 from the Conservatives, and added them to the independents (which went from 15 to 36).
The "working government majority" is listed as zero on both days on the Parliament's website, even though it was negative. I guess they do this by convention. Also, the Parliament website does not seem to consider the "confidence and supply" agreement as part of the math for the majority. It is listed as zero even for Jul 2, 2018, even though there were 316 Conservatives and 10 DUP back then (together that's more than half the 650 seats). So apparently they only consider a single-party or outright coalition a majority...
On 25 March 2014 (during the Cameron-Clegg coalition) they do list a working majority of 75. Actually the page back then had a more helpful footnote maths-wise:
Government majority calculated as Conservative and Liberal Democrat MPs less all other parties. This calculation excludes the Speaker, Deputy Speakers* (2 Labour, 1 Conservative) and Sinn Fein.