Look closely at the end of the roll with a strong light and perhaps a magnifier. Count the turns. Figure about 10" per turn as you get toward the end of the roll. You will probably err slightly on the safe side. It's all about pi * diameter.
For additional fun, run a paper strip around the roll making two corresponding marks at the start and end of the circumference. Measure the distance between the marks and note the corresponding amount of canvas actually run through the printer afterward. Add those numbers to an ever-growing empirical list that relates circumference to remaining media. Accuracy increases with every data point, it can be very gratifying and I used to feel just like a scientist when I did this. Don't knock empiricism.
My beloved 8400 measures remaining length automatically, even when I change rolls. So cool but now my unchallenged mind is slowly turning to mush.