I use the same settings, although mine points to opposite wall, so don't have too many "false" motion alerts. I've never really played with this setting myself, although if you notice you are not getting snapshots or motion triggers, you may have to increase the value of the threshold to decrease or increase its sensitivity.
I believe the "motion detected interval" is essentially how long it takes after the 1st motion trigger, so it won't keep triggering constantly within those 5s. You may want to bump this up or down depending on how much motion triggers you end up getting, such as traffic, people, windy trees or other items moving around throughout the day and night.
If you look at the DS2 settings via the browser via its IP, you will see how the 10% and 20% correlate, you may have to adjust it a big more or less. The nice thing is that it has a nice activity graph, so you can test and see how motion affects it and you will see it turn green and red and get an idea of how motions are measured. You may find it easier to use a laptop, so you can actually walk in front and test and see the results.
One you log in, you will see the hardware, camera, then internal camera to get to the settings and you can also adjust the box of the area you want it to trigger for (I blocked out the image for security):
I know that DS2 is a bit older now, but this is essentially an enterprise device, so there are all of advance settings you can play with. Hope it helps!