Looks like you're getting the grind to go from the tooth outer corner, right into the middle of the inside radius of the tooth.
The few times I tried with a hand file, getting that well placed angle, meant that the file was also notching the side plates. Close to the rivets.
Everything is stored in another county or else I'd try to find the chain that I tried filing a few teeth on and post a pic or two.
Oh and the chain was a 3/8 chisel tooth on .050 drive links. Probably an Oregon variant.