Estes Payloader II #3022 Build, Part 4, Fins, Leading Edge Rounding

The leading edge of the smaller dorsal fin is already rounded.
With a sanding block I'm sanding in a 45 degree "corner" into the larger main fin.
Don't sand any more off the dorsal fin. Going down the rounded edge, use your block to "knock" into the main fin balsa cutting the small 45 degree notch.

The leading edge of the main fins are rounded. Be careful when you get close to the 45 degree notch already at the fin joint, that is your "stop point".
After applying filler the angled notch probably won't be as sharply defined. That's okay, the transition between the two pieces will probably be smoother than you would get otherwise.

Looking ahead - 

The engine hook overhangs the back end of the tube.
If you've ever built a Estes Goblin, the rocket didn't stand up on its fins. It wobbled on the engine hook.
I'll recess the engine mount tube (not even with the main air frame BT-50 tube) so the model will stand on the fin trailing edges.