OK you've had no takers on this one so here's one way to file a perfectly straight and square edge.
First scribe a line that you want to to file down to. Next file down to about 1/32" above the line keeping the cuts as square as you possibly can. Now clamp a piece of square or rectangular metal bar perfectly in line with your scribe mark, leaving the 1/32" protruding. Finally file the remaining excess off while keeping your file pressed down flat on the bar of metal until all the excess is removed. You will now have a perfectly straight and square edge the you required.
Oops, I forgot to say..............WELCOME TO THE FORUM! :)