Photo time:
Below is illustrated the required components.
2- stair stops
2- 36” aluminum rulers (you can use longer rulers if you wish)
2- squeeze clamps (optional, but helpful)
Plus track, saw, etc.
I am setting up for a 16” cut. One facet of the hexagon is aligned with the 16” mark.
I lay one ruler atop the other to check that both are at the exact same point.
A close examination shows that they are NOT at the same precise point.
I make the required adjustment.
I clamp the two rulers to the panel to be cut.
Below shows how the bottom of the stair stop kisses against the panel to be cut.
This image shows the splinter guard kissing against the rulers. I am not using a Festool splinter guard. I believe it is a Makita version, and is black in color.
All that is left to do at this point is to clamp the track (if required), remove the rulers from the panel, and make your cut.
The extra step of confirming that both ends are exactly the same is optional. But it is an option that the parallel guides do not offer.
In my opinion, this is as accurate as the parallel guide, and for single cuts, probably just as fast. Multiple cuts using the parallel guide are much quicker however.
This is not a replacement for the squaring arm.
Feel free to ask any questions.
Post Script: You can unintentionally introduce parallelism errors by allowing the rulers to be skewed in an angle. I do find that it is pretty easy to keep the rulers square to the reference edge by observing whether the ends of the rulers appear aligned with the edge. For greater precision you can use a drafting triangle or a framing square to assure that the rulers are square to the reference edge.