In the middle. Usually, the middle of the contrast and brightness adjustments are optimal. The most important thing is to have adequate surrounding lighting to reduce the glare fromthe difference in relative light intensity. Also, good lighting also makes the keyboard and other documents easier to read and be sure to check those glasses for good focus!
It varies. Your best bet is to start with the factory settings and adjust as necessary. Decreasing brightness while increasing contrast may help, but the usual causes of eye strain are either dry eyes, focusing for an extended period, or the need for glasses. Consider using artificial tears just before and every so often while using the computer. If that doesn't help, see an eye doctor.