Can't you see those are safety sandals. And just like safety squints, are approved PPE across the whole 3rd world industrial sphere. OP will be perfectly safe.
For a machinist yep. For home gamers, a waste of money. They don't have the knowledge of where and when to use them nor the skills to get accurate repeatable measurements. So for OP's use whatever CCC, (Cheap, Cheerful, Chinese), caliper he's got is good enough.
It's the definition of "nominal size is what ever we say it is" that pisses me off. Buying wood/lumber is the worst offender of Nominal sizing, but even metals are getting worse. I used to buy a round bar of say, ASA1018 and it would be +0"/-.002". It's now +0/-.006", (that's +0/-.05mm and +0/-.15mm for those living in Boca Raton). At the end of my career as a toolmaker I was often forced to purchase oversized stock and waste time turning said stock into the actual sizes required.
I don't condone it, but I've done something similar once in a Boy Scouts meeting as a scout. Had sandals on and I don't remember the full meeting, but at one point we were chopping wood with an axe. I did it, but that's not my brightest move. There was also a time where I was getting my metalworking badge and entered the forge one time for a kinda free time work on your project thing with shorts on by mistake. Nobody stopped me in either case. This wasn't pre-2000s, so I'm not sure how I got away with either event considering safety is usually taken much more seriously nowadays.
My COVID home-improvement project was hardwoods. Something like 1000sqft. Never did it before. Did 90% of it wearing no more than basketball shorts and flipflops. W/e. Only a few toe injuries.