Determine the distance to the rafters on each side by measuring above the ceiling to the center of the exposed beam. Pic 1.

Add 3/4" to that measurement and measure the same way below the ceiling. This ensures your cut will be made along the approximate center of the rafter. Measure and mark with a pencil at several points, then draw through them to mark the cut lines you'll make right up the center of the ceiling rafter on each side. Pic 2.

Starting at the end of the rafter in the middle of the hole, measure outward to a point where the ceiling is undamaged. Mark it at the center and on each side, then draw a line through to intersect with the lines drawn up the centers of the side rafters. Pic 3.