"feel free to remove or link to another thread."

No I'd feel stupid doing the searching for you, it haven' been answered many a time ;O)

The question is, how many bells (options) can you hang off a x-mas three :? Suppose 20% for the 9 hours in office (6 over here), and 15 hours at 90% is 15.3 hours computing out of the possible 24 at 100% (less whatever actual use at mouse/keyboard). But,

1) BOINC is designed to back off when the user is at work, or the OS/scheduler needs CPU time at night... BOINC only uses the spare cycles.
2) BOINC can be set to pause for a predetermined amount of time till after stopping input (default 20 minutes I think, if client is left to compute based on preferences), in case you experience latency. E.g. 15 minutes, so when starting writing and then stop, the client stops the moment you started typing until 15 minutes after i.e. if you resume typing/mousing inside the 15 minutes, the client will restart the pause of 15 minutes.
3) BOINC can be set to pause if the user needs lots of CPU time (big speadsheet/render job). E.g. I've set my client's to pause if the non-BOINC load is greater than 50%. Such a pause is extended automatically in 10 seconds steps, thus if a heavy tasks lasts 70 seconds, the pause will be 80 seconds. (It's not so much that I want the pause because of my use, but some heavy BOINC projects break if not paused such as CEP2)

With the multithreaded systems, 4 - 8 is normal these days, the operating system will use 1 thread for your typing, really < 5% of that, and the rest for crunching, so even if you would specify for BOINC to only use 75% of processors on a quad, you'd be getting 24 hours computing * 3 and 1 thread reserved for your use. That's 72 hours crunching instead of 4 * 15.3 = 61.2 hours. Given that my well used laptop runs an average of 93% efficiency on all cores, that's 24 * 4 * 0.93 = 89 hours a day of computing... all without me knowing that BOINC is there wizzing away.

Power-conservation, just use the client scheduling function and set to crunch from e.g. 18:00 to 08:00, yes you can. Alternately, use scripting with boinccmd tool to change the allowed % of processors and the OS scheduling function to run 1 that sets low processors and another to sets high/all.