I look at the actual numbers when I am looking for trends in my lung function. I find its a much more accurate view of how I am doing. By looking at percent predicted you are putting a lot of trust in variables that could affect what you should be able to exhale. For example a dramatic weight loss or gain, using a different machine that may or may not be calibrated the same, or a lazy tech who uses your height and weight from your last visit rather than getting an accurate measurement that day.
Knowledge is power, most of us probably have great docs who really keep a close eye on things, but they are not infallible. In the end it is you who is responsible for your health. Personally, I ask them to print out my last PFT and then I compare it to the one I am currently doing, that way I can look at my own numbers. 9 times out of 10 my doc will come in and tell me that I was up or down from last time (information which I already know), and when he does that it just reaffirms my trust that he is in fact looking at my trends.
I'll reassert that knowledge is power, the more you know about your disease the better prepared you will be to fight it. Prevention is key with CF and catching a slowly declining FEV1 or FEV 25/75 may save you time in the hospital and lung function.