If you look at the Characteristic Curve you will see that the relation between density and exposure is a logarithmic one. The problem with test strips is that people are using an arithmetic series of exposures to determine proper exposure. This means that if your initial choice for the first step is far off the mark you are propably not going to get any valuable information from the first strip. For example if you do a strip of 1, 2, 3, 4, 5 seconds and the correct exposure is 20 seconds you are going to get a blank sheet. There are better methods than the usual test strip.