So to start from the beginning, your 25-75 are your small airways, which feel the biggest impact of CF. FEV1 is larger airways.
Think of the 25-75 as the tenths of a second on a stop watch, while FEV1 is the seconds.
The 25-75 measure is incredibly sensitive and as CF progresses, this # is the 1st to go. Which is why FEV1 is the most often quote #, and the most accurate predictor, of lung longevity.
If your FEV1 is 90s or above, the 25-75 is a tremendous measure to measure health, because you'll see more variability in that #.
So bottomline, As your FEV1 declines, it's not surprising to see much lower 25-75 #s. Although controlling allergies and inflammation can help the 25-75 (which I always applaud investigating allergies, as you know....), keep in mind that a low 25-75 can be a sign of the progressive nature of CF - permanent lung damage.
If you want to take a look at how to treat allergies, visit my blog under allergies. Take care.