If I understand your question, you're wondering how to make the second of Jerry Work's squaring legs. As I recall, Jerry used the upward facing t-track for his second leg.
To achieve a similar result, one could make both legs of the reference square use the holes in the tabletop. Those hole patterns are square to one another. Admittedly, there is a small loss of real estate. I think that loss is more than overcome by repeatability and flexibility.
Set up your reference corner legs one time and use them repeatedly anywhere you want on the tabletop. You might have some fine tuning issues but those can be handled a variety of ways.
If greater accuracy is required than the CNC machine that produced that tabletop, that's another issue and I think that deserves its own thread.
Tom