How best to？

The best setting value depends on the kind of a motion. Lower values retain sharpness in motion (good for dancing, fighting), higher values make an animation look smoother (good for calm motions like walking).
It's a field for experiment. You can even apply different settings to individual segments of the video.

If I use it again 10

Then spent over 5

It is now 5 or 15?

The trajectory filtering is at the value which is displayed.
In some cases it is not reapplied automatically. To reapply it manually, just touch the setting (press Smooth, then Sharpen).

thank you