Which jib tell tails are flying, upper, middle or lower? Ideally you want all 3 flying on a non spin boat but I take the middle, followed by the middle and uppers flying on a non-spin boat. On a spin boat I take the middle and lowers flying as you need the top twisted off to keep the slot open for the spinnaker. I tend to run jib downhaul loose except in super breezy conditions but even then only a little bit tighter than loose (by loose I mean wrinkles just out, maybe a little bit of wrinkles showing).

In general you want to release mainsail rotation and downhaul downwind but I find downhaul release less critical than rotation and general sail trim. I prefer downhaul coming off the mast directly and run to the crews trapeze ring as this gives you the ability to adjust while on the wire.

These are my tips for cats in general and they may not apply to the Nacra 5.8 in all conditions or even in most conditions as I have not sailed one before nor have I spend much time trying to make a beach cat go downwind with jib only.