DS2 - Backup Systems Points Cost

Assuming I'm correct that when a target receives a systems hit, all his
systems stop working (FireCon, Guidance, ECM, PDS, Stealth etc)
(Can someone correct me if I'm wrong here)

Then when calculating the points cost for backup systems - it's 30% of
the total cost for what exactly.  If the 30% takes into account the
guidance systems - how exactly do you work this out?

Thanks again

Paul Grogan
Runestone Games