I have a set just like the one you linked to. Mine is probably 30 or so years old. I have the one Starrett and a couple of other sets as well. I don't use any of them that often. These days, I use fixed blade precision squares. Here's one set I have: I have a 4 piece set (2", 3", 4" and 6") of Groz that are square to 16 microns (.00063"). I use them pretty much everyday I'm in the shop for wood, metal, setting my table saw blade perpendicular to the table - you name it. I also have a 12" machinist square that gets some use. It's pretty heavy, though. For bigger stuff, I use my trusty Woodpeckers 18" triangle. Way too pricey, but I've found it worth the money. https://www.woodpeck.com/onetime-too...ngle-2013.html It was one of their "One Time Tools" and no longer available. They's brought it back by popular demand 3 times since I bought mine, so it may come up for sale again.
Here's another triangle that's very similar to Woodpeckers, and a few dollars cheaper. https://tsoproducts.com/tso-precisio...asic-triangle/ It's accurate to within .001" over the full 18". Pretty good. They have some variations, but the price goes way up real fast for them. If I ever decide to buy another 18"er, I probably go with the TSO Products model.
The triangle is great for setting up the Festool circular saw guide rails for square cuts. TSO also has a product that clamps onto Festool guide rails and works nicely for setting up square cuts. I have the GRS-16. https://tsoproducts.com/tso-products...-rail-squares/
FWIW, I used my Woodpeckers square to do the rough gantry squaring when I mounted the gantry on my first machine and on my current one. My first one was within a few thousands on a 71" gantry. Beats the heck out of just guessing. I'm not too confident on this machine, though, I wasn't able to set up the triangle the way I wanted. That's okay. My squaring routine should get it dialed in nicely.
Gary