I've used the Kodak version for many, many years. It reliably gets the exposure into the ballpark, but I always have to make some minor adjustments. The adjustments are seldom as great as you describe, however. It may be that the plastic is absorbing UV or other light excessively with your particular setup and the Delta scale. First, try printing a variety of negatives using the scale to find the initial exposure. If the results are consistent (underexposure), you may be able to compensate and still have a useful tool. But remember, the scale only gets you in the ballpark.