According to the New Statesman, we should be looking for votes against between 120 and 150, assuming he will not lose. When scaled up, 121 equals those who voted against Major, 133 equals the proportion voting against May, and 147 represents those voting against Thatcher.
May and Thatcher were both forced to resign shortly thereafter – May announced her resignation within six months; Thatcher within 48 hours. Major clung on until the 1997 general election, an outcome no Tory MP can look back on creditably: the party lost 178 MPs of its 343 MPs.
Interestingly, the author, Harry Lambert, believes it is easier to get near to 180 than it was to get the required number of letters in. He reasons that this is a "for or against" vote. MPs cannot simply fail to support Johnson, as many have been doing.