I'm an outlier as regards FVC. Before my diagnosis my FVC was 68. It took about 9 more months for a diagnosis but my FVC was 80 at diagnosis (Mayo.) Then it went down to the mid-70s. I've been tested at four facilities (all very highly regarded) and my numbers are all over the place. I got so frustrated I bought my own testing equipment with a grant from ALS Guardian Angels.
I can measure FVC, slow vital capacity (which many ALS doctors prefer) MIP (inspiratory strength) MEP (expiratory strength) MVV (the amount of air you can move in 12 seconds, and much more. Although my FVC is never normal, it varies by time of day. Best before food. Same lying down as sitting up. My breathing muscles are still strong (MIP way above average.)
My only point is that other things need to be considered. I've been denied clinical trials because I've been diagnosed too long, I'm over 60, and/or my FVC was under 80. I do understand why but it's still frustrating.
FVC is just one number. Getting a Trilogy and cough assist and getting used to them is important. The cough assist can be used in reverse to expand the lungs and I have had success in increasing my lung volume by doing that.
There are so many factors to measure when talking about progression. I know one PALS who has never had any breathing issues yet he is totally paralyzed in all limbs. I know another who walked and ate up until the day he died from respiratory failure.
Also, progression speeds up, slows down, and even stops for months at a time.