Christian,
You didn't go from 70% to 48%. If the predicted values remain the same in both PFT visits, you went from 70% to 67%. I'm saying you have to compare apples to apples. You can't calculate your percentage based on two different predicted values and expect to be able to compare them. Gosh, I wish there was an easier way for me to explain it. Maybe re-read what I wrote in my first response. Also, all of my comments are based on the assumption that "what you blew" aka 1.7 Liters is your FEV1.
You did lose 1/10 of a liter, that's it. And based on a predicted value of 2.54 Liters, that's only a 3% loss. (70% down to 67%) Try not to get too hung up on the numbers. The biggest clue will be how you feel. Keep up with your airway clearance and nebs. Hang in there!