For going downwind without spi the most important thing is the jib trim. Let the jib out as far as you can while maintaing a round full sail (not flat because of letting it out too much). With this setup, find your fine line going as deep as possible while letting the leeward jib tales always flowing straight back. Now set up your main to fit the picture and steer to the jib tales, always on the edge of stalling the leeward tales, but never stalling them. This will be roughly 90 degrees of aparent wind, but not always.