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This is one way to get the time. http://www.freestylephoto.biz/41621-...on-Print-Scale
Another way is to have most of the paper covered by a card. Expose for 10 seconds, then move the card and expose for 10 seconds. Continue until the sheet is completely exposed. Ralph W. Lambrecht in his book describes how to do this by doubling the exposure time for each position. He is on APUG and the book is worth buying especially if you are just starting out.
After a while you will know that with lens A set at aperture B and paper C it take D seconds, is the starting point.