I think its hard to find a true ground near the boxster battery (just theorizing as i haven't really inspected the battery area) because of the cars high aluminum content. You need a nice "real metal" ground, or the negative pole of the battery.
I've never started a "dead" car without the donor car running and providing a charge to the dead battery. Usually let the donor car run a few minutes before trying to start the dead car.
How old is the battery that died? Do you know why it died? How was the "water" level in the dead battery? Given age and climate and use, it might be time to get a new battery. You'll know for sure that first cold fall morning when the battery is dead again
