Several people seem to think "BOINC is going to take too much of my machine for me to be able to do what I want to do quickly, so I'll throttle it." That's usually a bad idea and causes problems of it's own. I suggest the following:
1) Run everything at 100%. Only when you KNOW that BOINC is getting in the way should you try turning it down. All the BOINC tasks run at background priority, so everything you're doing should win anyway.
2) If find that you DO have a problem (which might include things getting too hot, now the CPU is running at 100%) the first thing to do is restrict the number of threads BOINC can use. While it seems to be sensible to restrict the percentage of CPU it uses, that's generally not a good idea because it's not a simple, smooth process. It's very lumpy and poorly behaved (because it's not being done by the OS, but by a low priority application).

If you have other software that runs at background but your machine somehow depends on it, then you might have an issue with BOINC. Always worth checking what else is running on the sea-bed ...