Couple things:
1. Start but rasping out "facets". Draw a continuous line 1/4 " down from the back of the bow. Draw a line down the center of the belly. Now rasp until you have straight facets: connect the lines. This will give you a roughly triangular shape. Then rasp the point (belly) flat. That will give you a trapezoidal shape. Now just keep working away with scraper, sandpaper, files, whatever, until the bow is round and the tiller even.
2. Get a gooseneck scraper. Google it for a picture. You can find these in most woodworking shops. On the other hand, you can make one by grinding and filing an old sawblade. Google the image, and then you can make your own: it will have the arc you desire.
