Ours does the same thing. Have not figured a way to get rid of it, but other problems include no cleaning during "near end", so don't leave town...
Best we have figured out, is to use a tape measure... It works pretty good, and is very reliable! (Make sure the printer has scanned the correct width by running a media compensation and see that it prints the whole width.)
Ours sometimes scans the wrong width. FlexiSign poll size has never worked for us, we always get comm error every time we try, and have to restart. It's no big deal to use the tape measure, we always have one by the printer.
Oh- And in relation to "near end"-messages, remember to pull out the cartridge, weigh it, and- if it weighs more than 430 grams, add 30 grams when putting it back in. That way you get to use up all the ink. If you wait untill empty, you will not be able to "add 30". It's about 8%, so at least $ 10 worth of ink each time you "add 30".