There is no good all around number to look for, as each of us has a different maxHR, resting HR, lactate threshold, etc, etc. To estimate your maxHR, you can use the formula 221-your age (so if you are 28, your estimated maxHR would be 193bpm). This, however, is proven quite well to not be very accurate. The best way to know your maxHr is to be tested for it or to test yourself to find this out--you can find directions on how to do this in many books (Cyclists Training Bible is one book to find that will help you do this). Finding your lactate threshold (LT) is harder and more involved, and can pretty much only be accurately determined in the lab if I'm not mistaken, as it requires blood tests. Determination of your LT will point you closer towards what is a good "cruising" HR% to not build up lactic acid your body can't get ride of. Hope this helps.
EDIT: beat to the punch again...I need to learn to type faster or something....