Just a look back in to my dim past. At the very start of my first Engineering course at collage we where all given a piece of black mild steel 11/2 inches thick by 2 by 2 inches with the instruction to file it flat and square on all sides. We where given a year to accomplish this task, though not one person managed to do it using hand tools though a lot of us came close. One bright spark gave a piece of steel the same size to his Farther who was a tool maker and he put it on a surface grinder with a perfect result.
I only mention this because as old school as it is it gave me the insight of how to file flat and straight, a skill worth its weight in Gold!!
