you really have to work with your driver to get spinnaker running down to an art.

Watch the luff of the sail. You want it to be just barely curling. Let out enough sheet to make that happen but just stay focused on that luff. If it curls too much it will collapse. If it starts to curl too much then pump that sheet once to get the curl out. After you trim, if you can't get the chute to re-fill, that means you're driving to high and you either need to drop the chute or steer down some.

Also another thing, if the boat starts to heel to much, don't let the chute out, let the driver steer down. (unless theres a boat below you on the course )