Cutting roof rafters for a new shed

Hi, I'm having some trouble cutting 2x4 rafters for my shed. Shed overall size is 9'x16', with a roof pitch of 5/12. I have a 2x8 ridge beam along the center. I cut a birdmouth into one 2x4 and it's an almost perfect fit on one side, but looks to be 1/2" short when I place on the other side of the ridge beam. How do people usually cut and install rafters? Do you cut them all to size beforehand (and hope for the best)? Do you cut the birdmouth and then see where the top should fall and cut in stages? Never did this before and it looks great on paper, but difficult when you're standing 12ft up and disappointing that all your design on paper doesn't work. Advice appreciated. Thanks Theodore