Begin by measuring across the front lentil and finding the center. Make a mark 3/4" to the side to give a guide for the edge of the 2 x 8. From here, mark a line every 16" across the front lentil. The rafters are going to span the distance of the lentils, and they're the pieces that will create the shade. And if it's shade you want from the rafters, they should run north and south. For this particular project, the rafters are spaced approximately 16" apart, which will supply good shade but still allow people underneath to see the sky.