The El Sereno Open Space Preserve is one of the best kept secrets in the south bay with great views and some challenging hiking trails. This isn't one of them; however, this short 1.2 mile trail maybe one you've never been on, but you do have a great view if you visit "A View for Shamar" (GCZCDJ).
It takes some doing to get a permit to park here (at least two business days before your planned date and having a fax machine is a big help). If you don't have a permit then you have no business coming up the private road, and bicycles are not allowed on the private road. The permit requires your signature (hence the fax machine) to acknowledge that you will be respectful of the neighbors and their road. Don't ruin it for the rest of us ... I like to be able to come back here again.
I used a Garmin GPSmap 60CS to average 300 readings (claims to be 5ft accurate), grabbed the coords from a Garmin Colorado (20 ft accurate at the time), compared to Google Maps ... made a reasonable compromise. You can park within 3 ft of the cache location, so don't go over the edge into the poison oak. The terrain difficulty is because you went through all the hassle of getting a permit, right? Otherwise the only other permissible parking can be found at the waypoints given by "ES02 El Sereno OSP (Nazgul Secret Trailhead #5)" ( GCTJEY) and hiking (or biking?) up Overlook Dr. to gate ES06 and traveling on the trail.