When I was in high school, my photography teacher taught us how to use 'test strips'. We'd take the strips of paper, put them under the enlarger, cover them with a piece of black cardboard, and expose for 5 seconds. Then we'd move the cover down exposing more of the paper, and then expose for another 5 seconds. Then again, then again, then again, until we had exposed the entire 'test strip'. She told us to choose the exposure that gave us the 'whitest whites and the blackest blacks', and each interval represented 5 seconds. I'm sure that this is quite the amateur way to do things, but wondered what, if any, the best method is to find the proper exposure time.