850 should be fine. I (and many others) use 500 as the low point for a few reasons. One is the "race to idle" theory, where in if you have short jobs that happen on occasion (a CPU wakeup to check on a file or what not) if that happens at 500Mhz vs slower, it completes faster and thus goes back to sleep sooner, saving power. There's also a tested and proven fact that the power vs cpu use efficiency is maxed out very close to 500Mhz. It's what the chip was designed to work at, and where most of the power-saving comes into play.

Also, FYI: 125 is in many cases unstable. Enough so that Nokia dropped it from their scaled listing, starting at 250 instead.

Same thing I do. Nothing wrong with asking questions when you're not sure what's going on. The three most useful and honest words in English are "I don't know". You should never be afraid to admit that, or to question what you know.