I sheared a rusty belt guard into five coupons and gave each one a different treatment. Times, costs, mass loss, and what each did to the metal underneath.
Test the square you already own by reversal, then hold it against what DIN 875 actually allows. Eight squares, my own measurements, one of them firewood.
A 1943 South Bend that showed up 78 tenths hollow at the headstock end. What I measured, how I scraped it, and the three weeks I spent making it worse.
You've read everything.