When something that should be square may not be!

[Mking]

Hi everyone, it's been a while since I posted but my project is slowly moving in the right direction. I'm midway through a welding course and spending probably too much time and money kitting out the garage and doing other prep. Anyway I took a punt on some set squares that were one of Lidl's offers this week. I know never to go cheap on tools but I have been pretty impressed by there things in the past and being German made I thought it worth a try. Anyway I thought I'd double check
their squareness when I got home. I held 2 together against a straight edge thinking that if they were perfectly square they would meet along their length. Sadly they didn't. They were only out by maybe 1/2mm over a 30 cm length but still it's an inaccuracy that will only be exacerbated over longer lengths. That prompted me to check the rest of my collection of squares which range in price and quality. None seemed perfect. Has anyone else encountered this? Have I just been unlucky, or am I being too exacting and need to stop worrying about such tiny errors? Has anyone got a suggestion for a guaranteed square if I need to be this exacting?