Old cotters are hard and can be reused. I do it. New cotters are soft, maybe not a great idea to reuse them.
For my technique, I coat the flat of the cotter with Sharpie. I put it in 98% there. I take it out. Where the Sharpie rubs off is a high spot. File there. Rinse and repeat.
Understand the principle. Already did that. Doubt I am attempting to execute anything at all like what you are doing. Just not getting the file to address only the indicated spots. It all gets cut off.
About the 98% part. What I am doing is tapping it once lightly with a hammer. Little enough it will come out just as easy to look at progress (hah!). Problem is that technique barely shows anything. To get a clear print it has to be fully assembled and test ridden. I am still hammering fairly hard on assembly. And then it is hard to get out and the threads mash.
I am not a mechanic. I have a head full of bicycle rubbish gathered over sixty odd years. Doesn’t mean my hands do it or my eyes see it. Need all the pieces plain.
Oh. How far, how deep, are we going? With each repetition the pin gets thinner. Or should it be such a fine file we get dozens of tries?
If you have old 9.0 pins I am buying.
And thank you.