Tories need 312 to keep a Conservative PM because DUP has ten and Sinn Fein 7.
They’re at 292 (-13 so far).
They can go as low as -18.
Any lower and Labor gets the PM.
OK at 300 now ... so by your calculation need 12 more.
The Sinn Fein MP’s never take their seats in Parliament so they are just a footnote in the election records.