I have placed the tip on finished paddles, but I find it easier to attach the tip before the blade is finished. I use a belt sander for much of the final sanding on the blade and this will also cut through the epoxy. If using hand tools to shape the blade, the epoxy will dull a sharp edge quickly.
Next, mark the finished outside tip and edge of the paddle on a non-stick surface. I used a piece of aluminum foil, but Saran Wrap or contact shelf paper (the kind to line cabinet shelves) works well too – I have problems with keeping the Saran Wrap wrinkle free and use contact paper more often.
The tip could extend any reasonable distance, but 3/8″ is my preference. I also wrap the epoxy around the paddle edge for about 6″, gradually tapering it to the blade.