I choose this setting because of two reasons: first: I need my CPU for other work as well, and second: I don't want my CPU to wear down too much by overstressing the cpu with max performance. To be honest I have no clue what I am doing, so if anyone says it does not work this way and my reasoning does not make sense at all, I love to be corrected.. So far it works for me though
Putting in 100% gave me no noticable more hashing power.
First: mention your CPU model. Then what kind of software you use on it and then if you use it 24/7 or only 8-10 hours a day.
There are many options - low power mining, undervolting, lowe the number of mining cores, mine only during the night, etc. If you are interested and tweaky enough, you can tailor it to match your lifestyle without affecting your performance.
But also keep in mind that a single CPU is not going to make millions of UPX. You can also write an article, or create a piece of software which will bring you much more UPX via the Bounty: https://bitcointalk.org/index.php?topic=5082311.20;topicseen