Just a quick pointer, you might have more luck posting this in the transplant section.
That said, the general rule is that CFers should be considered when they fall at or below 30%.
To answer your question in a more personal manner, I'm pre-transplant, but I am actively listed. I was listed at 35% FEV1, give or take, but I have massive infections and need IVs every few weeks to control them (I had 8 rounds of IVs in 2009, 5 the year before that, and have already had 2 in 2010). FEV1 isn't the only criteria they look at with CF -- my center has several CFers with lower FEV1s than me who are not actively listed, but in my case everyone (my CF doc, the transplant docs, the surgeons, etc) all agreed that it was definitely time. Doctors should look at O2 requirements, infection frequency and severity, lung function, antibiotic resistance,and overall quality of life when making the decision about whether or not to refer/list someone for transplant. There have been numerous studies showing that with CF, FEV1 is not a particularly good indicator of mortality, so a lot needs to go into the decision.