I always drill a small hole where the bottom of the notch is to be then cut down either side with a coping saw. Finish with sandpaper. (seems to work pretty good)
I also drill a hole with small round diamond hobby file but mine is drilled at the bottom of the nock after my initial cut.
The purpose is twofold !!! #1: It gives the split that you created nowhere to go by stopping the crack. I used it extensively while in the "Windshield Repair" business for 20+ years before filling the cracks with resin.
#2: If the hole is sized properly it should create the perfect size for your string serving and give you equivalently a snap on nock. Your string groove should be snug but not "tight", just a shade smaller than the hole at the bottom.
Lastly I taper and round the edges of the nock with a small single cut file and sandpaper before wrapping with sinew, staining, and sealing.