Some teach that there is a circular continuum, so that the far right is close to the far left. I remember this and my wife even has it in her 11th grade civics notes. But this is nonsense - continuums are linear. Does anything get so cold that it is hot? Have you ever heard of a guy who was so tall that he was a midget?
I think Marx and Engels captured the essence of leftist thinking. Their work represents an important milestone. Marx was an economist (a bad one) and he gave leftism a patina of “science.” To some degree, I see a continuum on the left depending on how much someone buys into Marxism.
Conservatives and libertarians have worked towards an intelligible theory devoid of Marxism. That is for intellectuals. I simply mean that as a descriptive statement without any attitude.
For many, “conservative” or "right-wing"simply means a rejection of one piece or another of current socialist indoctrination.
The press does not acknowledge the "left." If it has no name, it can not be attacked. But the press is happy to throw the "right-wing" label on anyone they don't like. It is a slander term.
Hitler, as we all have heard, was "right-wing." The fact that his party was the German National Socialist Workers' Party is never mentioned. Doesn't a "socialist workers' party" sound a little left-wing to you?
I have always heard that the left-right spectrum originates from the French Revolution; the ones who sat the furthest to the left were the anarchists, then to the right of them were the Jacobins. The furthest right were the monarchists, then the constitutional monarchists, then the constitutionalists.
This conception arose before Marx and Engels, yet due to their influence I would agree with you in using adherence to Marx and Engels as a benchmark for how Leftist an individual is; the only person who has nearly as much intellectual influence on the Left is Keynes.
The circular continuum theory is also bogus, and IIRC the academians today call it “Horseshoe Theory”. It has been gaining in popularity in recent days due to the establishment despising Trump, and on first glance despising Sanders (I believe Sanders’ campaign is carefully controlled by Hillary to test how far she can push the Overton Window to the left without backlash). One of their arguments is that both Trump supporters and Sanders supporters oppose free trade, and are more “isolationist” than the “mainstream politicians”. In the UK, they claim that UKIP and old-school Marxists like Corbyn are pro-Brexit, while the “center” is pro-EU.
The Horseshoe Theory proponents, of course, fail to admit that groups such as Trump supporters and Bernie supporters would disagree on such actions as how to go about dismantling NAFTA. A Trump supporter is interested in preserving social cohesion via incentivizing jobs to be located in America; a Sanders supporter would argue that NAFTA inadequately promotes Marxism, or perhaps would argue the anarcho-syndicalist/environmentalist viewpoint that NAFTA keeps us from getting rid of all technology and living in caves. When you compare anarcho-capitalists to anarcho-socialists, they would agree on “no government”, but they differ on what “no government” entails, how to reach a society with no government, and what form of government to settle on in a pragmatic sense.
So, when politics is reduced to “A supports X and opposes Y; B supports Y and opposes X”, the Horeshoe Theory seems appealing, yet WHY A supports X and the ultimate goal behind A supporting X... These are far more important.