Use Lith film and do an exposure under the enlarger of increments increasing in exposure time. You will have to do the math, but one way would be to expose for 10 seconds then start covering in steps and expose for 4 seconds, the 6 seconds then 8 seconds then 12 seconds. That will give you the series 10, 14, 20, 28, 40 seconds. You will have to use some trial and error. When you get one sheet of film that looks good, read off the densities just as you would an uncalibrated strip you bought.