Keep the flat the same angle, and file it so that when fitted the thread is just right through the nut. We used to have 3 sizes of flats, as some cheap cranks would stretch. Bear in mind I was in the 1950s, when bikes were still used for commuting, and were cheaper than walking, and there was no money for new cranks, the old stuff just had to work.
For the final fitting tap it in, firmly, and use the nut just to hold it in, not pull it in. If fitted correctly it does not need riding down the road and retightening.
British manufacturers always fitted cotter pins downwards with the crank to the rear, it was the Americans who decided the opposite. UK users had trouser clips, the americans needed chainguards.
Keith
For the final fitting tap it in, firmly, and use the nut just to hold it in, not pull it in. If fitted correctly it does not need riding down the road and retightening.
British manufacturers always fitted cotter pins downwards with the crank to the rear, it was the Americans who decided the opposite. UK users had trouser clips, the americans needed chainguards.
Keith