Just guess any number for the first one. If it is too dark half the time. If too light double the time. I use little pieces about 2.5 x 2.5 cm. You can zero in on the correct time that way. As you get closer reduce your multiplier from 2 to 1.4 or 1.26. Just keep putting the new pieces in the same place on the projected image. I have done it like that for almost 30 years. The problem with the other type of test strip is that it shows different exposures for different parts of the image which is of not much benefit to me.