Be careful: the angle here is not always right, and it is also not necessarily aligned with the laser gantry. These kinds of tricks are only useful for “good enough” rotational alignment, and not of much assistance in terms of placement.
The crumbtray is not uniformly orthogonal, and it can move/rotate inside your glowforge. Ergo, neither its position nor its orthogonality can be relied on for highly accurate applications.
As always the best way to do what you’re trying to ensure here is to install crumb tray boots to prevent the tray from moving, and then cut a corner jig as needed. It will give you absolute certainty of positioning and orthogonality as accurately as your machine can do. ( as for perfectly 90 degree angles, not all glowforges are perfectly square… in fact none of them are, there’s tolerances here.)