Here is something I wrote for a woodworking club magazine a number of years ago. It may be helpful.
No matter how much you spend for a device, you still don't know if it is square. I ran a large tool and die shop and we purchased a number of Brown & Sharp and Starrett devices and some of them were not "square". We had "standards" that our quality department periodically had validated by an outside service that we then used to verify the worker's tools.
One day, one of our designers brought in two plastic drawing triangles he had purchased at a local art supply store. He had them compared to our standards and they were as accurate as the tools could measure. The triangle cost a couple of dollars each. They would certainly serve very well as the "standard" in any woodworking shop to validate and/or adjust other devices.
An excellent way to validate the accuracy of the plastic squares is to use two squares on a flat surface. Get a $10-12 plastic 30-60-90 drafting square. To prove it's exactly 90°, take two to a glass counter, put the shorter legs on the counter and face the longer legs away from each other and butt them together (like a teepee). If the legs exactly butt, you can assume you have two perfect 90° angles. Using one of the plastic squares, do the same thing using your other tools. Any that mismatch, means that the tool is not square. You can also take the plastic square with you whenever you go to purchase another tool. Keep your "standard" somewhere where it doesn't get banged up.
Finally, remember that the wood you are using will expand and contract a couple of thousands from one day to the other. Does't pay to get too uptight.
While we are at it, I also only purchase the cheapest of adjustable squares. I square them with a drafting triangle and an auger file until they are square across 10". And I own a Bridge City square that isn't that accurate. Stainless steel machinist's squares are only square until you drop them. I have had several over the years and each has found a away to drop to the floor.
The key is to NEVER use your best square on for day to day measuring. Use it only as a reference tool to verify your other day to day tools.
In a comparison test reported in Fine Woodworking Magazine a couple of years ago, the Stanley 46-123 square was awarded the best value. It's much less expensive than a Starrett and just as accurate.