Op, I have been pooting off chunks of roids for days now just seeing what is there and I have some potential bad news. I'm hardly seeing any metal in metal rich or metallic. No palladium anywhere I look and I've never seen platinum at all... ever! It's starting to smell like mining may have got a nerf. of course it may also be I am having lots of luck... the bad kind.
HIP103107 about 18% to 55% per good roid
never say i didn't share
From my experiences mining which takes up alot of my playtime in elite other then exploring, to get the platinum group metals(palladium,platinum, ect) your looking for a pristine OR major reserves metallic asteroid field around a G to K class star.
Different stars seem to change the composition of asteroid clusters, red dwarfs usually giving uranite as their top metal/mineral, Brown Y or T classes tend to be rutile but i haven't mined much in them, if the roid clusters are left over accretion discs from the stars development it makes sense for the compositions to change.
When you get into the system pick any cluster in the belt you want say cluster 1, have a taste of the field (usually i test most of the roids unless its a large cluster, 10+ roids then i go nuts in the same cluster because your mostly guaranteed a good haul with a huge field) If you don't find the type of metal you want migrate to cluster 2 and taste again. Each cluster has a slightly different density of metals sometimes only one of the 7 or so clusters will have platinum group metals.
After that if you haven't found any
MOVE ON TO ANOTHER SYSTEM
I can't stress that enough if you've had no luck in a pristine metallic field for the last 20 minutes and have checked multiple clusters then the field probably does not contain the metals your looking for, Also if there are NPC;s about use them as mobile prospectors, as your going past to your choice of roid check what they have chipped off before they can scoop it up. It makes tasting a field easier and faster.
Oh and leave the poor NPC miners alone, if they're mining "your" roid remember you both have a "quota" of chunks per roid which won't change no matter how many miners there are on the roid at the time!