There isn't a friction feed device on the planet that will feed material without some deviation. I'm not sure how packages other than Flexi accommodate this but I'd bet the rent that they do.
In Flexi all you have to do is, in the Production Manager, select the setup that you want to correct then do Setup->Output Size Compensation. This brings up a dialog where you can specify both the expected length and the measured length. Whatever you input in these text boxes will be normalized to 12". Different media can have different compensation. For example, If I'm printing 36" banners I'll set it to 72 expected and 71.125 measured. If I'm doing 3M180C I'll do 60" and 50.5".
In Flexi you can output a test print from this dialog but it's only 12", a little small for any sort of precise correction.
One caveat, the corrections aren't cumulative. If you enter a correction that proves to be insufficient, you can't add to it, you have to enter the entire correction. A little fundamental Algebra and you easily can figure out a cumulative correction, but you have to do the math, not the program.