I have the DH damper too in a coil and find it pretty good.
Slow Speed compression is for just that slow speed hits when the fork is moving slow + on the slow gives more resistance on slow speed travel.
High Speed is for high speed hits – like riding into a rock or a big drop – when the fork is moving fast. + on the fas tgives more resistance to the high speed travel.
Stick them to about +4 clicks on both and you will be fine.
Like I said about the spring – try one less – normally find them floating on here for about 25 – 30 quid. Especially of your not landing big drops etc I think there is definite room to run a lesser spring rate. How is your sag?