Brian from Monument
I used something like that 5 cut.

But in the end the only thing that matters is the proof of the squareness.
That can only come from cuts.
So why not start with that method.

The OP laid out a sound way in geometry and drafting of constructing a perpendicular line.
Another post laid out a slightly flawed way using the saw blade.
I don't use the saw blade. As any heel (hope I have the right term) in the blade
will result in the fence being out of square. If the blade is not absolutely parallel to the mitre slots. That method wont work. Rely on the kerf. After all the where the blade is is only part how the cut goes.
The sled is using the mitre slots.

So the kerf is king! Long live the kerf. (and the 5 cut method is part of it)